Atomic force microscopy and electron microscopy with energy dispersive X-ray analysis was used to characterize changes in teh structure and composition of La0.8Sr0.2MnO3 (LSM) nanoparticles supported on single crystal YSZ (100) (yttria-stabilized zirconia) and SrTiO3(100) surfaces as a function of temperature and exposure to oxidizing and reducing environments. On YSZ(100), LSM particles were found to decompose into Mn- and La-rich phases and spread over the surface upon calcination in air at temperatures above 1123 K. The Mn-rich phase was observed to have a higher mobility and spread more rapidly. In contrast to YSZ(100), on SrTiO3(100) the LSM particles underwent agglomeration via an Ostwald ripening mechanism upon calcination at temperatures above 1123 K, resulting in an increase in the particle size. Phase separation was not observed on this substrate.