A round robin test was performed to determine the reliability of values for the glass transition temperatureTg as determined by DTA on polymers. Ten different instruments were involved. The test material was high molecular weight polystyrene. Values forTg (midpoint) were reported in the range 107°C±2 K. The respective heat flow curves differed considerably in shape. In the literature aTg of 100°C is often given for polystyrene. The discrepancy between this value and the value of 107°C found in the round robin test is due to three differences: the thermal history of the sample, the evaluation of the heat flow curves, and the effect of finite sample size.