The thermally induced strain response of unidirectional P100S/AZ91D carbon fibre-reinforced magnesium composite was studied over five cycles in the +/- 100 degrees C temperature range. A temperature-dependent one-dimensional model was employed to predict the anticipated response to the cycling thermal environment. Strain hysteresis was observed during cycling and attributed to matrix yielding. First cycle residual plastic strains were modelled with reasonable agreement. Experimental results deviated from predictions during subsequent cycles with continued thermal ratcheting shifting the hysteresis loops to higher strains with increasing cycles. This was thought to be associated with interfacial debonding and frictional sliding at fibre/matrix interfaces. The effect of thermal treatment on composite expansion behaviour was investigated and the results discussed in terms of minimising thermally induced deformations during anticipated service conditions. Treatments were found to affect the first cycle behaviour, reducing in particular residual plastic strain generation. Matrix yield strength was exceeded over the thermal cycle due to a lack of sufficient hardening, and since interfacial conditions were unaltered, interfacial sliding and thermal ratcheting could not be eliminated. The potential for improvement of C/Mg composite thermal strain response was explored in the light of the current findings.