Photocatalytic-photothermal evaporators are emerging as promising water purification technologies capable of removing both inorganic and organic pollutants while offering low-cost and low-energy desalination. Despite their potential, the interplay between photocatalysis and photothermal evaporation remains poorly understood. Here, we investigate the sequential application of the two processes and demonstrate, for the first time, the existence of synergistic effects that are independent of the specific materials used. We show that pollutant rejection by evaporation is very effective on the aromatic intermediates formed via hydroxyl-radical attack – an insight of general relevance to advanced oxidation processes. However, we also identify p-benzoquinone as a critical volatile intermediate whose concentration remains significant in the distillate after conventional liquid-phase photocatalysis combined with evaporation. By examining the role of the water matrix, including common inorganic electrolytes and non-volatile organic compounds, we further reveal conditions under which the combined process becomes practically ineffective – an issue not previously recognized for photocatalytic–photothermal systems. Building on these findings, we propose an improved treatment sequence in which the evaporation step is brought forward, mitigating the inhibitory effects of non-volatile species and enabling robust synergistic coupling. This work highlights the overlooked importance of gas-phase photocatalysis and provides a rational framework for the future design and optimization of photocatalytic-photothermal evaporators.