Reaction-volume dependent photocatalytic reduction of nitrobenzene was investigated in various batch reactors and continuous-flow microreactors (CFMs) with different surface area to volume ratios (SA/V) under visible-light irradiation, leading to control of the photochemical reduction route, reaction speed and product selectivity. A batch reactor with less than 1.5 cm(-1) SA/V ratio produced only aniline in 96% selectivity with 40% conversion by a direct reaction route. In contrast, the CFM with an 80 cm(-1) SA/V ratio showed production of high-value azoxybenzene in 99% selectivity with 99% conversion through a condensation reaction route. In particular, a parylene thin film microreactor with a 100 mu m channel height (400 cm(-1) SA/V ratio) achieved perfect conversion and selectivity to azoxybenzene within only 50 min of reaction, compared to 20 h of reaction in the batch reactor to produce aniline. The mechanistic reaction progress was monitored by conducting time-dependent reactions of nitrobenzene and nitrosobenzene. Furthermore, the photocatalysis of various nitro-aromatic compounds also exhibited the identical reaction-volume dependent chemistry with excellent product selectivity.