Selective vibrational excitations of OH stretching modes of methanol-pyridine hydrogen-bonded complexes trapped in solid argon or nitrogen at 7 K were carried out in the range 3400-3280 cm(-1). This proved an efficient way for inducing isomerization processes within heterodimers and larger aggregates. The main photoproducts are non-H-bonded species, with OH and CO frequencies, respectively, close to 3665 and 1020 cm(-1). These unbonded species are unstable, with lifetimes strongly temperature dependent in the range 7-20 K. The possible structures of the heterodimer have been calculated theoretically according to a two-step method. The potential energy surface is firstly explored using a semiempirical method, then the properties of its minima are calculated in the framework of the density functional method. The deepest minimum corresponds to the quasilinear N . . . HO hydrogen-bonded structure, with vibrational properties in good agreement with the matrix data obtained after deposition at low concentration in both dopants. Several other minima were examined in which H bonding is either weak (OH... pi electrons) or nonexistent. The vibrational perturbations are weak in all cases, but the identification of the photoproduct of the stable dimer to the form involving the OH... pi interaction can be discarded. The stable forms of the mixed trimers were also calculated. (Methanol)(2)-pyridine has a cyclic structure, the methanol dimer being tied to pyridine through a strong OH . . .N hydrogen bond and a weak CH . . .O interaction. Methanol-(pyridine)(2) is also cyclic, with a OH . . .N bond slightly stronger than in the heterodimer. Comparison with experimental data allows identification of this heterotrimer in nitrogen matrices, in the presence of an excess of pyridine with respect to methanol. Among its photolysis products, one class in which the OH group is perturbed on the oxygen atom has been identified.