Abstract: It was found that in 0.75 mol·L-1 H2SO4 solution, Mo6+ was reduced to Mo5+ with ascorbic acid, and a complex between Mo5+ and NH4SCN was formed with its maximum absorption wavelength at 465 nm. Based on these facts, the determination of Mo6+ by flow injection (FI)-spectrophotometry was proposed. Linear relationship between values of ΔA and mass concentration of Mo6+ was kept in the range of 3.3-93.3 g·L-1, with detection limit (3s/k) of the method of 0.35 g·L-1. The proposed method was used in the determination of Mo6+ in sample of maceration extract of cobalt molybdenum catalysts, and values of recovery found were in the range of 95.0%-101%, with RSD (n=6) less than 1%.