The thermochemistry of H3O+(X)(n) clusters (X = H-2, N-2 and CO and n = 1-3) was determined by equilibrium measurements using the mass selected drift tube technique. The binding energies for H3O+(H-2)(n) (n = 1 and 2) are 3.4 and 3.5 kcal mol(-1), and for H3O+(N-2)(n) (n = 1-3) are 7.8, 7.3 and 6.3 kcal/mol, respectively. The binding energy of the polar CO molecule is 11.2 kcal mol(-1), and ab initio calculations show that the H2OH+center dot center dot center dot CO conformation is more stable than the H2OH+center dot center dot center dot OC conformation. Saddle point energy of 17.6 kcal/mol is calculated for inversion between these conformational isomers. (c) 2006 Elsevier B.V. All rights reserved.