Minimum energy geometries, harmonic vibrational frequencies, and stepwise binding energies have been obtained for the cluster ions NO+.(H2O)(n), n = 1-4. From systematic ab initio calculations an the lighter NO+.(H2O)(n) complexes (n = 1-2) at MPn, CCSD, and CCSD(T) levels of electron correlation with different basis sets, it was found that the MP2/6-311++G(2d,p) level of theory was reliable for the calculation of minimum-energy geometries and harmonic vibrational frequencies. Relative electronic energies were evaluated at the MP2/aug-cc-pVTZ//MP2/6-311++G(2d,p) level. The inclusion of zero point energy (ZPE) corrections, as well as counterpoise corrections for basis set superposition errors (BSSE), in the calculation of binding energies was essential to obtain the correct energy ordering for the different isomers of a cluster ion. The nature of the stepwise hydration processes was discussed based on the isomeric structures obtained. A reaction route for nitrous acid (HONO) formation when a water molecule is added to NO+.(H2O)(3) has been established.