During the process development of montelukast sodium, three polar impurities and one non-polar impurity with respect to montelukast sodium were detected by simple reverse phase high-performance liquid chromatography (HPLC). Initially, all the four impurities were identified by the liquid chromatography-mass spectrometry (LC-MS) data and out of four impurities, three have been prepared by the synthetic method and remaining one is isolated by preparative HPLC. Based on the spectral data (IR, (1)H NMR, (13)C NMR and MS), the structure of these impurities 1-4 were characterised as 1-[[[(1R)-1-[3-[(1E)-2-(7-chloro-2-quinolinyl)ethenyl]phenyl-3-[2-(1-hydroxy-1-methylethyl)phenyl]propyl]thio]methyl]cyclopropane acetamide (impurity-1), {1-[1-{3-[2-(7-chloro-quinolin-2-yl)-vinyl]-phenyl}-3-(2-isopropenyl-phenyl)-propylsulfanylmethyl]-cyclopropyl}-acetic acid (impurity-2), 1-[[[(1R)-1-[3-[(1E)-2-(7-chloro-2-quinolinyl)ethyl]phenyl-3-[2-(1-hydroxy-1-methylethyl)phenyl]propyl]thio]methyl]cyclopropaneacetic acid (impurity-3) and 1-[[[(1R)-1-[3-[(1E)-2-(2-quinolinyl)ethenyl]phenyl-3-[2-(1-hydroxy-1-methylethyl)phenyl]propyl]thio]methyl]cyclopropaneacetic acid (impurity-4).