Different interleaving procedures for high speed digital transmission are studied. The effect of multiplexing on the line signal is considered. Special attention is paid to digital sum variation and runlength properties, and also to spectral content for timing recovery. Multiplexing/demultiplexing synchronisation procedures have been examined. Emphasis has been placed on 5B6B, although some results can readily be extended to other block line codes of the familymBnB.