In this work, an effective and low-cost method for the determination of sodium or potassiumdiclofenac is proposed in its pure form and in their pharmaceutical preparations. The method is based on thereaction between diclofenac and tetrachloro-p-benzoquinone (p-chloranil), in methanol medium. This reactionwas accelerated by irradiating of reactional mixture with microwave energy (1100 W) during 27 seconds,producing a charge transfer complex with a maximum absorption at 535 nm. The optimal reaction conditionsvalues such as reagent concentration, heating time and stability of the reaction product were determined.Beer’s law is obeyed in a concentration range from of 1.25x10 -4 to 2.00x10 -3 mol l -1 with a correlationcoefficient of 0.9993 and molar absorptivity of 0.49 x10 3 l mol -1 cm -1 . The limit of detection (LOD) was1.35x10 -5 mol l -1 and the limit of quantification (LOQ) was 4.49x10 -5 mol l -1 . In the presence of the commonexcipients, such as glucose, lactose, talc, starch, magnesium stearate, sodium sulphite, titanium dioxide,polyethyleneglycol, polyvinylpirrolidone, mannitol and benzilic alcohol no interferences were observed.The analytical results obtained by applying the proposed method compare very favorably with those givenby the United States Pharmacopeia standard procedure.