Metal oxide nanoparticles are essential in various applications, and the synthesis through gas-phase generation methods offers a rapid and reliable pathway for nanoparticle production. Yet achieving precise control over their formation remains challenging due to the complex nature of oxidation processes. While bulk oxidation states can be assessed via off-line measurements, the dynamic nature of surface oxidation is more difficult to monitor and optimize in real time. Here, we investigate the surface oxidation state of unsupported tin oxide nanoparticles using an aerosol sample-delivery system and in-flight X-ray photoelectron spectroscopy. This powerful method allows the continuous monitoring of the surface oxidation of the gas-phase generated nanoparticles in real time, avoiding uncertainties associated with postcollection alterations. Tin oxide nanoparticles are widely used in gas sensing and catalytic applications, where the surface oxide layer plays a crucial role in determining their performance. Our findings demonstrate how the surface oxidation state of the free-flying particles can be controlled by adjusting the carrier gas composition, in-flight heating temperature, and particle composition. Specifically, the surface oxides of tin are partially reduced when heated in a slightly reducing atmosphere, and the reduction is further enhanced by forming mixed tin-gold nanoparticles. While previous studies on metal oxide nanoparticles have focused predominantly on bulk properties or off-line analysis, this study employs real-time in-flight X-ray photoelectron spectroscopy to investigate details of the surface oxidation state. Understanding the surface oxidation of metal oxide nanoparticles is essential to optimize processes, such as in-flight coating or subsequent deposition into a protective environment. This approach enables the exploration of direct correlations between generation conditions and surface properties, providing valuable insights into optimizing gas-phase nanoparticle synthesis. © 2025 The Authors. Published by American Chemical Society.