AbstractPrecise heat capacity values are reported over the temperature range from 10 to 360°K. for a sample of atactic polystyrene having a narrow molecular weight distribution. This sample was taken from the stock from which National Bureau of Standards Standard Sample 705, Narrow Molecular Weight Distribution Polystyrene, was established. Data are reported for the sample as received, and after an annealing procedure. At temperatures below about 60°K. a systematic difference comparable with the limits of experimental precision appears between the values obtained for the present sample as received and after the annealing, although at higher temperatures the values for the two conditions showed no systematic difference beyond the limits of precision of the measurements. At temperatures above 100°K., previously published values for atactic polystyrene samples of various molecular weight distributions and for isotactic polystyrene agree within about 0.5% of the values from this investigation. At temperatures below 100°K. significant heat capacity differences appear, especially between values for the atactic and the isotactic isomers, and even between atactic samples of different molecular weight distribut