<p>Future space missions, such as lunar exploration, will require lubricants with enhanced antiwear performance and extended operational lifetimes compared with currently available space lubricants. To address this challenge, this study investigates the applicability of widely used phosphorus- and sulfur-containing antiwear additives under high-vacuum conditions for space lubricants. However, the performance and tribochemical behavior of such additives under vacuum conditions lacking oxygen and moisture remain insufficiently understood. In this work, nine antiwear additives containing phosphorus and/or sulfur were dissolved in poly-α-olefin (PAO) base oil, and sliding tests were performed under oil-immersed conditions in both air and high-vacuum conditions. Although most additives exhibited increased wear under vacuum, some additives maintained stable antiwear performance despite the absence of atmospheric oxygen and moisture. The tribochemical behavior on the sliding surfaces was analyzed by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) to elucidate the underlying tribochemical mechanisms. The results suggest that, under the present test conditions, the acidic phosphate ester with saturated alkyl chains maintained antiwear performance under vacuum, and sulfur-containing species may contribute to the protection of fresh surfaces in oxygen-deficient environments. Furthermore, the tribofilm compositions were found to differ between air and vacuum conditions. These findings provide useful implications for further studies on the molecular design of lubricants for future space missions.</p>