A Simple and rapid spectrophotometric method has been developed and validated for the determination of Adefovir (ADE) bulk and formulations. The proposed method is based on nucleophilic substitution reaction of ADE with 1,2- naphthoquinone-4-sulphonate (NQS) in an alkaline medium to form an orange-colored product of maximum absorption peak (
max) at 450 nm. The stoichiometry and kinetics of the reaction were studied, and the reaction mechanism was postulated. Under the optimized reaction conditions, Beer's law correlating the absorbance with ADE concentration was obeyed in the range of 1–7μg mL−1. The limits of detection and quantitation were 0.3 and 0.8μg/ mL−1 respectively. The proposed method was successfully applied to the determination of ADE in its pharmaceutical tablets with good accuracy and precisions; the label claim percentage was 99.56 ± 0.45 to 99.92 ± 0.20.