Statistical mechanics Monte Carlo simulations of the proton transfer in [H3NHNH3]+ in aqueous solution have been carried out using the combined quantum mechanical and molecular mechanical AM1/TIP3P potential. As in the gas phase, squeezing and stretching motions of the donor–acceptor distance have profound effects on the proton transfer. Solvation increases the activation free energy of the proton transfer by 3 kcal/mol over that in the gas phase. The results suggest that the combined QM/MM potential may be used in models that treat the proton motion quantum mechanically and the solvent dynamics classically. © 1993 John Wiley & Sons, Inc.