Folks, I found some of the documentation in GHC.Prim confusing - so I thought I'd share. The documentation for the ByteArray# type[1] explains that's it's a raw region in memory that also remembers it's size. Consequently I expected sizeOfByteArray# to return the same number that I passed in to newByteArray#. But it doesn't - It returned however much it decided to allocate, which on my platform is always a multiple of four bytes. This is something which could be clarified in the documentation. So it turns out I need to carry my own length around in something like (Int, ByteArray#). (I tested all of this with the primitive[2] package, which is a thin wrapper around GHC.Prim) Antoine 1: http://hackage.haskell.org/packages/archive/ghc-prim/0.1.0.0/doc/html/GHC-Pr... 2: http://hackage.haskell.org/package/primitive