diff --git a/eth/common/eth_types.nim b/eth/common/eth_types.nim
index 28fbd93..840a7c6 100644
--- a/eth/common/eth_types.nim
+++ b/eth/common/eth_types.nim
@@ -1,5 +1,6 @@
 import
-  stew/endians2, options, times, chronicles,
+  strutils,
+  stew/[endians2, byteutils], options, times, chronicles,
   stint, nimcrypto/[keccak, hash], eth/rlp, eth/trie/[trie_defs, db]
 
 export
@@ -125,6 +126,7 @@ type
     receiptRoot*:   Hash256
     blockNumber*:   BlockNumber
 
+  # TODO: Make BlockNumber a uint64 and deprecate either this or BlockHashOrNumber
   HashOrNum* = object
     case isHash*: bool
     of true:
@@ -132,6 +134,13 @@ type
     else:
       number*: BlockNumber
 
+  BlockHashOrNumber* = object
+    case isHash*: bool
+    of true:
+      hash*: Hash256
+    else:
+      number*: uint64
+
   BlocksRequest* = object
     startBlock*: HashOrNum
     maxResults*, skip*: uint
@@ -301,6 +310,24 @@ proc read*(rlp: var Rlp, T: typedesc[HashOrStatus]): T {.inline.} =
     raise newException(RlpTypeMismatch,
       "HashOrStatus expected, but the source RLP is not a blob of right size.")
 
+func init*(T: type BlockHashOrNumber, str: string): T
+          {.raises: [ValueError, Defect].} =
+  if str.startsWith "0x":
+    if str.len != sizeof(result.hash.data) * 2 + 2:
+      raise newException(ValueError, "Block hash has incorrect length")
+
+    result.isHash = true
+    hexToByteArray(str, result.hash.data)
+  else:
+    result.isHash = false
+    result.number = parseBiggestUInt str
+
+func `$`*(x: BlockHashOrNumber): string =
+  if x.isHash:
+    "0x" & x.hash.data.toHex
+  else:
+    $x.number
+
 proc append*(rlpWriter: var RlpWriter, value: HashOrStatus) {.inline.} =
   if value.isHash:
     rlpWriter.append(value.hash)
diff --git a/eth/common/eth_types_json_serialization.nim b/eth/common/eth_types_json_serialization.nim
index 24a910f..6b027cf 100644
--- a/eth/common/eth_types_json_serialization.nim
+++ b/eth/common/eth_types_json_serialization.nim
@@ -2,6 +2,9 @@ import
   times, net,
   json_serialization, nimcrypto/[hash, utils], eth_types
 
+export
+  json_serialization
+
 {.push raises: [SerializationError, IOError, Defect].}
 
 proc writeValue*(w: var JsonWriter, a: MDigest) =
@@ -42,3 +45,12 @@ proc writeValue*(w: var JsonWriter, value: HashOrNum) =
     w.writeField("number", value.number)
   w.endRecord()
 
+proc writeValue*(w: var JsonWriter, value: BlockHashOrNumber) =
+  w.writeValue $value
+
+proc readValue*(r: var JsonReader, value: var BlockHashOrNumber) =
+  try:
+    value = init(BlockHashOrNumber, r.readValue(string))
+  except ValueError:
+    r.raiseUnexpectedValue("A hex-encoded block hash or a decimal block number expected")
+
diff --git a/tests/common/test_eth_types.nim b/tests/common/test_eth_types.nim
new file mode 100644
index 0000000..ebdf10c
--- /dev/null
+++ b/tests/common/test_eth_types.nim
@@ -0,0 +1,51 @@
+import
+  unittest,
+  nimcrypto/hash,
+  serialization/testing/generic_suite,
+  ../../eth/common/[eth_types, eth_types_json_serialization]
+
+func `==`*(lhs, rhs: BlockHashOrNumber): bool =
+  if lhs.isHash != rhs.isHash:
+    return false
+
+  if lhs.isHash:
+    lhs.hash == rhs.hash
+  else:
+    lhs.number == rhs.number
+
+suite "BlockHashOrNumber":
+  test "construction":
+    expect ValueError:
+      var x = BlockHashOrNumber.init ""
+      echo "An empty string should not produce the value ", x
+
+    let x1 = BlockHashOrNumber.init "0"
+    check((not x1.isHash) and x1.number == 0)
+
+    let x2 = BlockHashOrNumber.init "1241328"
+    check((not x1.isHash) and x2.number == 1241328)
+
+    expect ValueError:
+      var x = BlockHashOrNumber.init "0x"
+      echo "An empty hash should not produce the value ", x
+
+    expect ValueError:
+      var x = BlockHashOrNumber.init "0xff11"
+      echo "A shorter hash should not produce the value ", x
+
+    expect ValueError:
+      var x = BlockHashOrNumber.init "0x7a64245f7f95164f6176d90bd4903dbdd3e5433d555dd1385e81787f9672c58z"
+      echo "An invalid hash should not produce the value ", x
+
+    expect ValueError:
+      var x = BlockHashOrNumber.init "0x7a64245f7f95164f6176d90bd4903dbdd3e5433d555dd1385e81787f9672c58811"
+      echo "A longer hash should not produce the value ", x
+
+  test "serialization":
+    let hash = Hash256.fromHex "0x7a64245f7f95164f6176d90bd4903dbdd3e5433d555dd1385e81787f9672c588"
+
+    Json.roundtripTest BlockHashOrNumber(isHash: true, hash: hash),
+                       "\"0x7a64245f7f95164f6176d90bd4903dbdd3e5433d555dd1385e81787f9672c588\""
+
+    Json.roundtripTest BlockHashOrNumber(isHash: false, number: 1209231231),
+                       "\"1209231231\""