The nucleation and stability of metal nanoparticles on oxides are of great importance in catalysis, gas sensors and microelectronics. Using variable temperature and pressure scanning tunneling microscopy (STM) in conjunction with in-situ deposition techniques, a versatile imaging approach has been developed that allows the nucleation, growth and alloying of individual nanoparticles to be followed in-situ. Selected nanoparticles can be exposed to reactive gas mixtures and the evolution of their morphology followed while spanning a pressure range of over eleven orders of magnitude. Since the size and composition of the particles can be controlled individually, direct comparison of various particles with similar treatments can be carried out in a single experiment. In particular, the stabilities of Au and Ag nanoparticles supported on TiO@sub 2@ (110) have been investigated while carrying out a catalytic reaction (CO oxidization) and while exposing the sample to an aqueous environment.