A comprehensive, micromechanical model, relating the longitudinal stress and transverse strain during tensile testing of unidirectional fibre toughened ceramic matrix composites, has been developed. The model uses different unit-cells to describe the composite and considers all relevant damage as it develops throughout a tensile test. Specifically, the proposed model takes into account the Poisson contraction of fibre and matrix, the redistribution of mechanical stress and the relief of thermal stress due to the development of damage, and the build-up of compressive radial stresses at the interface due to the radial mismatch between fibre and matrix after interface debonding and sliding. Consequently, the modelled transverse strain response depends on a wide variety of structural and mechanical parameters. The followed approach has been assessed by comparing the simulated and experimentally observed response of a unidirectional SiC/CAS composite. Theory and experiment are in excellent agreement for an experimentally determined set of constituent properties, but a parametric study shows the important effect of some of these, such as the radial interfacial mismatch, which are difficult to determine experimentally. (C) 1997 Acta Metallurgica Inc.