The cluster size and structure dependence of local microsolvation environment on intramolecular energy transfer and unimolecular dissociation is studied for the systems H2O2-Ar-N (N = 1-39) using the method of classical trajectory simulation. The rates and mechanisms of intramolecular energy transfer and molecular decomposition of the complex are investigated as a function of N. Three major mechanisms for cluster modification of isolated molecule behavior are operative : vibrational deactivation of the excited molecule, modification of intramolecular energy redistribution, and cluster-induced recombination. The evolution of these effects is studied as cluster size and structure are varied. It is found that the degree of solvation of the hydrogen peroxide molecule, rather than the absolute cluster size, determines the extent of the cluster effects.