In this study a C18 reversed phase cation exchange mixed mode HPLC method using 10 mmol/l of TFA, acetonitrile and corona charged aerosol detection for the purity control of carbocisteine was developed and validated. The method was capable of separating and quantifying the main related compounds of carbocisteine. We furthermore studied the stability of the impurity N,S dicarboxymethylcysteine and confirmed the cyclization product of this impurity.