Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO2 suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k(Fe(VI))[Fe(VI)](1.25) where k(Fe(VI)) = [Ammonia]/a[Ammonia]+b), a = 6.0 x 10(3) mu M-0.25 s as and b=4.1 x 10(6) mu M (-1.25)s(-1). The rate constant, k(Fe(VI)), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).