An alternative method for accomplishing direct rate measurements for reactions on a millisecond time scale is proposed, based on the continuous addition of reagent at a constant rate to the species to be determined using very simple instrumentation. This technique is suitable for routine analyses and was evaluated using two classical fast reactions, i.e., the complexation of iron(III) with thiocyanate and the oxidation of L-ascorbic acid with 2,6-dichlorophenolindophenol. Theoretical equations have been derived and the simulated and experimental kinetic curves were found to be consistent.