Fe(III)-citrate complexes are photoreactive and ubiquitous in natural waters. In this study, the effect of Fe(III)/citrate concentrations and ratio on the photoproduction of •OH as a function of pH (3–9) was systematically investigated. The •OH formation mechanism was elucidated according to the pH-dependent formation rate of •OH and the speciation distribution analysis of Fe(III) species. At high Fe(III)-to-citrate ratio (10:50), the •OH photoproduction increased with decreasing pH. In contrast, the •OH photoproduction increased in the order of pH 9.0 < 3.0 < 7.0 < 6.0 < 5.0 at low Fe(III)-to-citrate ratios (10:100–10:300). At identical Fe(III)-to-citrate ratio (1:10), high concentration of Fe(III)-citrate complexes rendered a downward trend for •OH production with increasing pH. FeOHcit– is the predominant reactive species responsible for the •OH formation at high pH. The optimal pH for •OH production was governed by the amount of O2• – and the stability of Fe(II) species in the Fe(III)-cit solution. The Fe(III)-cit-induced photodegradation of diphenhydramine verified the pH-dependent trend for •OH production. By GC-MS and LC-ESI-MS analyses, the photoproducts of diphenhydramine were identified and the degradation pathway was proposed.