Mercury (Hg) bound to fine aerosols (PM2.5-Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM2.5 samples collected between 15 September and 16 October 2015 from Beijing, China, among which 26 were collected during daytime (between 08:00 and 18:30 LT) and 30 during night (between 19:00 and 07:30 LT). The results show that diel variation was statistically significant (p 0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in 1199Hg values resulted primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between 1199Hg and (i) 1201Hg (positive, slope = 1.1), (ii) δ 202Hg (positive, slope = 1.15), (iii) content of Hg in PM2.5 (negative), (iv) sunshine durations (positive) and (v) ozone concentration (positive) observed for consecutive day–night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM2.5-Hg.