The metastable proton-bound dimer of acetonitrile and methanol, (CH3CN)(CH3OH)H+, exhibits two unimolecular reactions on the microsecond time scale, a simple bond cleavage reaction to form CH3CNH+ and CH3OH, and the loss of water to form CH3CNCH3+. The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH3CNCH3)(H2O)(+). Collision-induced dissociation mass spectrometry was used to identify the two sets of reaction products, and the results of isotopic labeling experiments suggest that there is a rate-limiting isomerization step going from the proton-bound dimer to the second complex. The competition between the two channels was modeled with ab initio calculations and RRKM rate theory to obtain relative energies for the reaction surface. The transition structure for the rate-determining isomerization could not be located with ab initio calculations, and so its relative energy was estimated using RRKM theory. The 0 K binding energy of the (CH3CN)(CH3OH)H+ complex was calculated to be 121 kJ mol(-1) at the G2 level of theory (relative to the dissociation products CH3CNH+ and CH3OH).