Classical Monte Carlo and molecular dynamics (MD) simulations were carried out to investigate the structures, the infrared spectra, and the rigid-nonrigid transitions of small methanol clusters (CH3OH)n for n = 3-6. The study was motivated by experimental results for these clusters from size specific infrared (IR) dissociation spectroscopy, The MD simulations revealed the following transitions : The trimer passes from a rigid ring configuration into a series of nonrigid open chain structures starting at 197 K. For n = 4 and 5 such transitions occur between rings and rapidly fluctuating ring structures at T = 357 and 243 K, respectively. For n = 6 first a pure isomeric transition between the two energetically lowest isomers of S6 and C2 symmetry is found at 35 K, and then a similar transition to a nonrigid behavior as is observed for n = 4 and 5 is seen at 197 K. The measured spectra display in all cases the rigid lowest energy configurations.