Solvation of the Cs+ ion in a binary mixture of acetone and methanol is studied in mixed cluster ions of the form Cs+[(CH3)2CO]N(CH3OH)M. Vibrational predissociation spectra are recorded for mass-selected clusters with well-known solvent composition. The pure acetone cluster ions Cs+[(CH3)2CO]N do not absorb in the infrared within the range of our CO2 laser, leaving the CO stretch of methanol as the sole chromophore for excitation. Monte Carlo simulations of the heteroclusters are performed using pairwise additive intermolecular potentials. The infrared spectra indicate that a distinct change in cluster structure takes place in Cs+[(CH3)2CO]N(CH3OH)M cluster ions between a size of N = 3, M = 1 and N = 4, M = 1, as evidenced by a large shift in absorption frequency. No other significant change in cluster structure is noted through a cluster size of N = 12, M = 1. Spectra of clusters with M > 1 indicate that the methanol molecules occupy spectroscopically distinct sites. The simulations suggest that the distinct change in cluster structure is due to the formation of a hydrogen bond between an acetone and a methanol molecule before the first solvent shell is completely filled. A comparison of the spectra of the heteroclusters and those of Cs+(CH3OH)N and the (CH3)2CO-CH3OH dimer support this assertion. The sensitivity of Monte Carlo simulations to the choice of intermolecular potentials is discussed.