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)-O-center dot as a function of pH (3-9) was systematically investigated. The (OH)-O-center dot formation mechanism was elucidated according to the pH-dependent formation rate of (OH)-O-center dot and the speciation distribution analysis of Fe(III) species. At high Fe(III)-to-citrate ratio (10:50), the (OH)-O-center dot photoproduction increased with decreasing pH. In contrast, the (OH)-O-center dot 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)-O-center dot production with increasing pH. FeOHcit(-) is the predominant reactive species responsible for the (OH)-O-center dot formation at high pH. The optimal pH for (OH)-O-center dot production was governed by the amount of O-2(center dot-) 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)-O-center dot production. By GC-MS and LC-ESI-MS analyses, the photoproducts of diphenhydramine were identified and the degradation pathway was proposed.