In this work an experimental method is proposed for the first and pseudo- first- order reactions which are readily followed by uv/visible spectroscopy. The method is based on the measurement of differential light absorption, ΔA of the two identical reactions simultaneously which are initialized seperately at different times (time interval: Δt) in a double- beam spectrophotometer. The present method was successfully applied the pseudo- first- order oxidation of KI by K₂S₂O₈. Keywords :