A serial data decoder is described for decoding interleaved variable length and fixed length codes. The decoder includes a state machine which moves between a branching hierarchy of states in dependence upon the bits of received serial data. When the state machine reaches a state corresponding to a complete variable length code having been received, it then enters a delay state lasting for the length of time necessary to receive the following fixed length code prior to return to a reset state. The state machine can be implemented using a static ram 10 storing data including pointers for controlling the movements between states.