The thermal expansion behaviour of several fibre-reinforced PEEK composites is assessed. It is shown that thermal expansion behaviour is consistent, and changes in a predictable manner with changes in fibre type. Using a composite manufactured such that no interfacial bonding took place, it is demonstrated that compressive forces caused by differential thermal contraction of fibre and matrix are sufficiently large to dominate behaviour in a direction parallel to the fibres. This suggests that PEEK composites should be resistant to changes in thermal expansion behaviour with repeated thermal cycling, and such resistance is demonstrated for AS4/PEEK (APC-2/AS4). It is shown that conventional models for predicting laminate response from unidirectional composite properties are valid for such materials, but it is also shown that the common analytical models for calculating transverse fibre behaviour from composite properties are inaccurate.