In this paper we report on the isotopic effect on the cage-induced excited-state quenching inside small Ar-m clusters (m<10(2)) solvated in large Ne-N clusters (Napproximate to7.5x10(3)). Excited OH(A)/OD(A) fragments are produced by photodissociation of H2O and D2O molecules and the quenching agents are correspondingly H or D atoms. The decrease of the fluorescence yield with the size of the cluster m>m(0) is observed in both cases and it is attributed to the formation of the cage of argon atoms around the doped molecule. Interestingly, more atoms are needed to induce the fluorescence quenching of OD*(A) fragments, m(0)=21+/-3, compared to the electronically excited state quenching of OH*(A) molecules, 11+/-2. A diffusion model containing two free parameters, the quenching cross section sigma(q) and the number of argon atoms forming the cage m(0), explains the effect in terms of the residence time of the hydrogen atom inside the cage. We suggest that the melting of the doped rare gas clusters is responsible for the different predissociation dynamics. The quenching cross section obtained from the experimental data is in good agreement with former experiments. (C) 2004 American Institute of Physics.