Background:The reported chromatographic methods for N-acetyl cysteine [NAC] are reverse phase HPLC and ion pair chromatography [IPC] for related substances test in bulk and in formulations. No reported stability indicating methods for the estimation of related substances in NAC effervescent formulation was found in literature. Objective: The present work was aimed at developing a selective, sensitive and reproducible stability indicating high-performance liquid chromatographic method for the quantitative determination of known, unknown impurities, degradation impurities and processrelated impurities of NAC effervescent formulation. Method: A reversed phase ion pair chromatographic method was developed employing Cadenza C18 column as the stationary phase and 0.01M octane sulphonate [pH 2.20], methanol and acetonitrile in the ratio 90:8:2 as the mobile phase. A gradient programme was followed with a run time of 55 minutes. 0.3 M hydrochloric acid was selected as the optimum diluent. The performance of the method was validated according to the ICHQ2R1 guidelines. Results: The method was found to be linear from 1.5 to 25µg/ml for impurities A, C and D and from 2.0 to 25 µg/ml for impurity B. The official impurities C and D were mapped in all stress conditions. Additionally, impurity B was also seen in acidic conditions. Conclusion: The results from the study demonstrate that the method is suitable for evaluating the stability of NAC effervescent tablet. Key words: N-acetyl cysteine, Reverse phase HPLC, Effervescent formulation, Ion pair chromatography, Related substances,Validation. Key message: An ion pair chromatographic method was developed for quantifying the related substances of N-acetyl cysteine effervescent Tablets. Selection of diluent was an important variable in the method development.