90 degrees cross ply epoxy resin laminates reinforced with either glass or carbon fibres were tested in tension as a function of ply thickness. Transverse cracking of the 90 degrees plies and longitudinal splitting of the 0 degrees plies has been observed and the experimental strains recorded. Energetics arguments have been presented which explain this behaviour and permit experimental ply cracking strains to be estimated. The effects of thermal strains generated during fabrication and Poisson generated strains have been assessed and included in the energetics calculations. The thermal strains were determined and found to be large in carbon fibre/epoxy laminates compared with glass fibre/epoxy laminates. Poisson effects are significant in the glass fibre/epoxy laminates due to the higher failure strain of these composites. The micro mechanics of failure were also studied and fibre debonding identified as a source of 90 degrees ply failure.