Uniaxial thermal contraction/expansion tests were carried out below 40 degrees C on poly(styrene-co-acrylonitrile) (PSAN) and acrylonitrile-butadiene-styrene (ABS) specimens. In PSAN, contraction and expansion were reversible and linear with temperature. By contrast, some ABS materials underwent an irreversible volumetric expansion over a temperature interval of similar to 30 K, which was superimposed on the ordinary thermal contraction. On subsequent reheating and cooling in the temperature range - 80 to + 40 degrees C, expansion and contraction in these ABS polymers followed an approximately linear path, but after annealing at 105 degrees C the specimen again exhibited an S-shaped thermal contraction curve. It is concluded that the anomalous contraction behaviour observed during initial cooling is due to cavitation of the rubber particles, which results in a relaxation of thermally induced tensile stresses within the rubber phase, and a corresponding fall in the coefficient of expansion of ABS. Analysis of thermal expansion data provides a method for estimating the volume fraction of rubber particles that have cavitated, and shows that void formation is controlled by an energy barrier. The data suggest that the void formation energies required to surmount this barrier are comparable with those of the naturally occurring voids that constitute the free volume. The effects of rubber particle cavitation on yielding were studied by cooling ABS specimens to sub-zero temperatures, and then subjecting them to creep tests at 23 degrees C. It was found that cooling caused a progressive increase in the creep rate, which was ten times faster in ABS cooled to -60 degrees C than in uncooled ABS.