Thermal expansion coefficients between 10 C and 60 C were determined in 10 degree increments with 10 plastic filling materials and two plastics used for protheses (3 specimens each). The coefficients became larger as the temperature increased. The composites with 20 to 60.10(-6)/C are closest to the thermal expansion coefficient of the tooth; unfilled plastics with 80 to 130.10(-6)/C, the farthest removed. Plastics containing less anorganic filling material demonstrated thermal expansion coefficients between 70 and 90.10(-6)/C. Water absorption did not appear to influence the expansion.