Poly(ether ether ketone) (PEEK) is increasingly used in selective laser sintering (SLS) due to its superior mechanical and thermal properties and biocompatibility. It is well known that the properties of selective laser sintering (SLS) powder govern the processability and quality of the parts. The SLS printing process exposes the PEEK bed/feedstock to variable elevated temperatures (above 300 degree C) for a prolonged time. The effect of this thermal history on the part quality is unknown. This work evaluates the changes that occur in thermally treated PEEK powder through spectroscopic, morphological, and rheological characterizations. It was found that crystallinity increased marginally within the first two hours, but particle size and shape remained virtually unchanged. The thermal treatment also resulted in a significant increase in the melt viscosity of the PEEK powder, and its flowability slightly improved. Understanding these changes could help to reduce waste through recycling of some portion of PEEK powders used in SLS printing.