Infrared induced isomerization of methanol-acetonitrile mixed aggregates trapped in argon and nitrogen matrices has been studied using an optical parametric oscillator for monochromatic irradiations at the OH and CH stretching frequencies of complexed methanol. Several forms of the 1:1 complex have been characterized by most of their vibrational modes; two (Ar) or three (N-2) hydrogen-bonded forms are observed after matrix deposition; upon irradiation they are converted into another H-bonded and one non-H-bonded species. This unbonded species is highly unstable, with a lifetime of the order of 24 min in argon at 7.2 K. In the presence of an excess of acetonitrile, larger aggregates are identified, with OH stretching frequencies weakly redshifted with respect to those of the 1:1 complex. They are also infrared photosensitive, giving rise to non-hydrogen-bonded structures. The possible structures of the heterodimer have been calculated theoretically. The deepest minimum of the potential energy surface is the quasilinear H-bonded form, with vibrational properties which agree with the matrix data obtained after sample deposition. As for the photolysis product, its vibrational spectrum is compatible with a secondary minimum with a pseudocyclic structure in which the OH group is perturbed on the oxygen atom.