The crystallization of synthetic trans-1,4-polyisoprene (TPI) was carried out in 0.05 to 20.0% (wt/vol) hexane, octane and amyl acetate solutions and the melting temperature, T-m, of the resulting crystals was measured by differential scanning calorimetry (d.s.c.). The crystal form and T-m of the crystals were independent of the concentration of the solutions. On crystallization at low temperatures, TPI crystals appeared before the TPI solutions reached the desired crystallization temperature, T-c, and the T-m was independent of the T-c. This phenomenon was quantitatively analysed with respect to the conversion temperature and the crystallization conditions. In contrast, at high temperatures, TPI crystals were observed after the TPI solutions reached a given T-c, and the T-m was dependent on the T-c.