Selective vibrational excitations of OH stretching modes of methanol aggregates trapped in solid nitrogen were carried out in the range 3500-3370 cm(-1). This proved an efficient means of interconverting trimers and possibly branched polymers absorbing in this domain. Irradiation in the absorption band of the cyclic trimer (CT) around 3450 cm(-1) leads to a cycle opening and to the appearance of several groups of bands assigned to different forms of open chain trimer (OTi, i = 1-3). Irradiations at the absorption frequencies of OT1 and OT2 interconverted these open species and partially regenerate CT. Isotopic dilution experiments enable to improve vibrational analysis of the open trimers, previously advanced from data obtained with irradiations performed with polychromatic light [Chem. Phys. Lett. 217, 123 (1994)]. Two other bands at 3424 and 3370 cm(-1) growing upon annealing as the high polymer bands around 3250 cm(-1), also turn out to be sensitive to selective irradiation. OH/OD and O-16/O-18 isotopic substitutions lead to the conclusion that the species (X) responsible for these absorptions is a branched trimer generated by fixation of one extra molecule on a cyclic trimer. Among the various products of photolysis one unstable species, with a lifetime of 8 min at 7 K, has been identified as a branched open chain trimer with one methanol molecule acting as proton biacceptor. Isomerization kinetic studies were followed in the nu CO region. According to the nature of the precursor, more or less complicated conversion schemes were identified. A;quantitative analysis was carried out in only two cases, namely the photolysis of CT and OT1 upon irradiation at 3450 and 3389 cm(-1), respectively. In both cases the kinetics were interpreted in term of first order processes, leading to quantum yield values between 10(-2) and 10(-3). (C) 1997 American Institute of Physics. [S0021-9606(97)01741-8].