This paper describes an investigation of the conditions affecting the determination of phosphate using the reduced phosphoan-timonylmolybdic acid method. The aim was to develop a determination method with faster kinetics than the original procedure of Murphy and Riley, for automation using flow-injection analysis. Optimum colour formation was found to occur at [H+] [MoO2-4] ratios between 50-80 at all pH values tested (0.36-1.06). The maximum rate of formation occurs at a sol[H+] [MoO2-4] ratio of 70 within a pH range of 0.57-0.88 when an antimony concentration greater than 0.06 mM and ascorbic acid concentration greater than 0.009 M in the final solution are used. Full colour development occurs within 0.8-1 min. The ascorbic acid reagent was found to be stable for 30 days. The results of the study indicated that by suitable selection of reagent conditions, rapid chromophore development can be achieved. © 1995.