In this study, we systematically investigated various crystal-to-crystal transitions relating to poly(L-lactide) (PLLA) cocrystallized with low-molecular-weight compounds using wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. First, the solvent exchange and the resultant crystal transition of solvent complexes were investigated. Basically, the solvent exchange treatments at -25 degrees C became successful, although some specific phenomena depending on solvent species were seen. The ratio of the a-form in the crystalline region increased by an increase in the solvent exchange temperature. Second, the crystal transition behavior between CO2 and solvent complexes was investigated. The complete transition from solvent complex to CO2 complex was observed for all solvent complexes. In contrast, it was found that types of solvents and the surrounding temperature have a great effect on the transition behavior from CO2 to solvent complexes. Finally, the guest-induced transitions of noncomplex crystals (alpha-, alpha'-, and alpha ''-forms) were examined. As a result, it was revealed that the guest-induced transition behavior of noncomplex crystals was much influenced by the order of crystal (chain conformation and packing) of noncomplexes (alpha, alpha', and alpha '') as well as kinds of guests.