The phase transformation of form III isotactic poly(1-butene) was investigated as a function of temperature. The polymer was isothermally precipitated from a dilute solution in iso-amyl acetate and observed with real-time synchrotron small- and wide-angle X-ray scattering techniques. The results confirmed that the polymorphic transition of form III was strongly dependent on temperature. The phase transformation from form III to form I' proceeded at a temperature of ca. 80 degrees C. This was accompanied by the presence of two distinctly different lamellar periodicities arising from form III and I' crystals, respectively. The coexistence of form III and I' crystals can persist up to 103 degrees C, followed by melting and recrystallizing into form II crystals. Finally, the reflections resulting from form II crystals disappeared at 118 degrees C.