The reaction NO+(H2O)3 + H2O --> (H2OH+ + HONO is considered to be an important source of proton hydrates in the ionosphere. Although kinetic measurements have been made on the steps leading to the formation of NO+(H2O)(3), the above reaction has never been observed directly. Here we present the results of an alternative approach to the problem, where the behavior of mixed cluster ions of the form NO+(H2O)(n) is examined. These species could be considered as collision intermediates in reactions leading to the formation of proton hydrates. For reasons which are explained below, the most detailed experiments were undertaken on NO+(D2O)(n) rather than NO+(H2O)(n) ions. The results show that the reaction NO+(D2O)(n) + D2O --> (D2O)(n)(+) + DONO, becomes significant for n = 3 and is a dominant reaction path for n = 4 and beyond. Similar experiments on the system O-2(+)(D2O)(n) were not successful.