Objectives. A simple test method was developed to determine the coefficient of thermal expansion of prevailing restorative resin composites and to study the transient behavior as a function of temperature and repeated thermocycles. Methods. Strain gauges were used to determine the thermal expansion for seven commonly used restorative resin composites by measuring the instantaneous strain along with temperature change. The temperature was measured by means of a thermocouple, the tip of which was embedded in the composite. The differences among the test groups were analyzed using ANOVA, followed by Scheffé's multiple comparisons test. Results. The coefficient of thermal expansion determined for the composites tested was: 22.5 ± 1.4 × 10-6/° (Z-100), 23.5 ± 1.4 × 10-6/°C (P-50), 32.6 ± 1.6 × 10-6/°C (Herculite XR), 34.1 ± 1.8 × 10-6/°C (APH), 35.4 ± 1.4 × 10-6/°C (Conquest), 41.6 ± 1.5 × 10-6/°C (Silux Plus), 44.7 ± 1.2 × 10-6/°C (Heliomolar). The coefficient was almost linear in the considered temperature range (26-75°C) for all composites (r > 0.99) and decreased with each consecutive thermocycle (p < 0.1). Significance. Thermally induced loads, introduced into restored teeth by the mismatch of the coefficient of thermal expansion of the tooth and the restorative material, may be related to microleakage and wear problems. A highly filled hybrid composite such as Z-100 had a coefficient of thermal expansion closest to that of the tooth crown, confirming other studies which demonstrated the benefits of high filler loading in matching the properties of the dental hard tissues.