The mechanism of thermoreversible gelation in solutions of isotactic poly(methyl methacrylate) (iPMMA) was investigated through the study of the temperature-concentration relations, the melting behavior of the solvent and the gel, the change in molecular conformation by FTIR spectroscopy and structural analysis by wide angle X-ray scattering. Such a gelation proceeds very fast below -20 degrees C while crystallisation from solution necessitates an induction period of several weeks, even at high degrees of undercooling. The molecular conformation is different from the one in crystalline samples. The mechanism consists of different steps: a coil to (double) helix transition followed by an intermolecular association that finally leads to a macroscopic crystal lattice. The data clearly illustrate that such a crystalline state is reached by a mechanism that is different from the well-known folded chain crystallisation. (C) 1999 Elsevier Science Ltd. All rights reserved.