The charge-transfer complex formed by the interaction between the electron acceptor, p-chloranilic acid and an electron donor diethylcarbamazine was adopted for the assay of the drug in pure powder and in tablets. In dioxane-chloroform medium, chloranilic acid was found to form a 1 : 1 molecular complex with diethylcarbamazine with maximum absorption band at 540 nm. At 540 nm, the absorbance was linear (γ=0.9999) over the concentration range of 10-400 μg/ml. The pertinent equilibrium constants and molar absorptivities were calculated at different temperatures using the Scott equation and were found to decrease with increase in temperature. The free energy change (ΔG°) and the enthalpy of formation (ΔH°) as well as the entropy (ΔS°) were determined for the various interactions. The proposed method has been conveniently applied in the analysis of commercially available diethylcarbamazine citrate tablets with good accuracy and precision.