A method for evaluating compounds as nucleating agents in isotactic polystyrene (IPS) is described that involves measuring the polymer heat of fusion by differential scanning calorimetry after a prescribed thermal history. Twenty-nine compounds are described that increase the overall crystallization rate of three IPS samples. In general, the effective compounds contain an aromatic nucleus, are polyfunctional, and are high melting. Most are organic nitrogen heterocyclics. The most effective nucleating agents are thymine, melamine, oxamide, 2-hydroxybenzimidazole, and 4-hydroxy-6,7-dimethylpteridine. The use of plasticizers such as butyl stearate, diisobutyl adipate, or paraffin oil with nucleating agents often results in a synergistic increase in crystallinity.