By means of both a density balance and gradient tube, experimental density data are obtained to illustrate the thermal crystallization behavior of 3 aromatic polyesters at temperatures from 90°C to 150°C. The data are discussed with reference to both amount and rate of crystallization and density curves are presented to show qualitative similarity between kinetics of polymer crystallization and the kinetics of chemical reactions. From the experimental data, a temperature, for convenience labeled the minimum crystallization temperature, is described and is shown to be 30° to 50° above the second‐order transition temperature for the polyesters. In addition to thermal crystallization, data are presented to show that immersion media may initiate crystallization below the minimum temperature associated with thermal crystallization of polymers. Liquids used as immersion media for polyethylene terephthalate were water, methyl alcohol, acetone, benzene, nitromethane, and nitric acid. The last four liquids initiate crystallization of the polymer at room temperature.