The ability to reuse feedstock in additive manufacturing powder bed processes is vital for cost control and widespread adoption of the technology. This requires an understanding of the effects of thermal cycling in the ambient conditions an additive machine has on the physical, chemical, and microstructural properties of the powder. Specifically, the work reported here centers on characterization of the changes that occur due to thermal cycling with 316L stainless steel, 17-4 PH stainless steel, and Nylon 12 powders. in a custom rig or an industrial selective laser melting (SLM) machine. All powders used in industrial machines exhibited shifts towards larger particle sizes as they were used. Of other properties measured, Nylon 12 showed no change. The surface chemistry of the used stainless steel powder is consistent with that of gas atomized powder with only the spatter having evidence of increased oxidation. The spatter was also used to determine the events leading particle to agglomeration and these observations were numerically modeled to represent the measured particle size distributions.