[en] A new method for the simultaneous determination of N-acetylcysteine and its pharmacopeial impurities, cysteine, cystine, N,N'-diacetylcystine and N,S-diacetylcysteine in an effervescent tablet has been developed. The method is based on on-line LC-UV-MS using a pneumatically-assisted electrospray interface (ionspray). The stability of the thiol moieties of the analytes was ensured by the acidic pH of the LC mobile phase. Quantitation of N-acetylcysteine was performed with UV detection to avoid ion-source overloading effect due to its higher concentration, whereas the impurities could be easily separated and quantified in MS. The method was validated in terms of stability, linearity, precision and accuracy.