Joint source-channel decoding of variable length codes (VLC) for image and video transmission over wireless links is a subject of increasing interest. This paper proposes an optimum decoder of VLC sequences which exploits inherent redundancy in source data, namely the VLC syntax and source constraints. The proposed decoder is able to deliver both Maximum Likelihood optimum hard-output and soft-output solutions. The complexity of the algorithm is analyzed and a reduced complexity version of this algorithm with a slightly sub-optimal performance is proposed. An iterative joint source-channel decoding system is designed for demonstrating the performance of the proposed soft-input soft-output (SISO) decoder. Realistic contexts have been used for simulations: typical video sequences and typical radiomobile channels.