Vibrational relaxation and energy redistribution of Zundel cations H5O2+ in acetonitrile and neat water are studied by femtosecond infrared pump-probe spectroscopy with resonant excitation of the 1200 cm(-1) proton transfer mode. Transient spectra in a range from 1000 to 2200 cm(-1) reveal intermode anharmonic couplings and a sub-100 fs decay of the proton transfer mode, followed by femtosecond vibrational redistribution within the cations' vibrational manifold and picosecond energy transfer to the solvent. Energy transfer to an aqueous environment occurs on the same time scale as within neat water, demonstrating the strong coupling of cations and water solvation shell.