Abstract <p>A binary mixture of isoniazid and mesalazine was analyzed by the application of the H-point standard addition method. The method is based on the difference in absorbance at the wavelength pair (∆λ) of 540 and 605 nm for the charge-transfer complexes formed between the above drugs as n-donors and o-chloranil as a π-acceptor in a basic medium. The results showed that isoniazid and mesalazine can be determined in their mixture, considering isoniazid as the analyte and mesalazine as the interferent at a weight ratio of 6.0 : 20.0 and 8.5 : 10.0, with recoveries from 94.3 to 105% and a relative standard deviation of less than 4.3%. Under optimum conditions, the suggested method was successfully applied to the simultaneous determination of isoniazid and mesalazine in synthetic and pharmaceutical formulations.</p>