A fast-flow reactor quadrupole mass spectrometer coupled with a laser vaporization source is used to study the gas-phase reactions of nickel oxide cluster anions (NixOy)(-) with nitric oxide (NO). The results show that nitrogen dioxide (NO2) is formed on the nickel oxide clusters and that this formation leads to the loss;of one or more species from the clusters in order to dissipate the heat of formation. The species lost from the clusters are nickel (Ni) and nickel oxide (NiO). Also, there is evidence that the association species of nitric oxide with nickel oxide anions rapidly form NO2. Additional experiments were conducted in order to determine if this NO2 formation mechanism is a function of how the nickel oxide cluster anions are formed. It was observed that a significantly different reaction occurs when nickel clusters are allowed to oxidize at room temperature and are then reacted with nitric oxide.