To obtain three-dimensional structural information about growing single crystals, a FTIR microscope for the oblique transmission method was developed. A newly designed optical system enabled oblique irradiation from any direction of incidence with high efficiency. This system was employed for the study on the crystallization process of n-C36H74. The in-situ IR measurement carried out on a growing single crystal showed that a polytypic transformation took place during the solution crystallization of the M-011 modification. At the inception of crystallization, single crystals were of the single-layered polytype. The double-layered polytype was generated immediately through a heterogeneous nucleation on the single crystals, and then grew through the solution-mediated phase transition from the single-layered to the double-layered polytype. At the end, single crystals changed completely to the double-layered polytype. (C) 2002 Elsevier Science B.V. All rights reserved.