Four new 6-methoxy-tetrahydro-beta-carboline derivatives (1-6-methoxy-1-phenyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indole, 2-6-methoxy-1-(4-methoxyphenyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indole, 3-6-methoxy-1-methyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indole and 4-2-methoxy-4-(6-methoxy-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indol-1-yl) phenol) were prepared via the Maillard reaction using 5-methoxytryptamine and various aldehydes in water. The synthesis reaction conditions were optimised in catalyst loading, temperature and time using LC-MS for optimum yields. Surface response methodology and contour plot was selected as an approach for optimisation. The optimum yield could be achieved below 50 degrees C within 5 h at 7 mole % catalyst loading at yields > 70%. The beta-carboline compounds produced were characterised using electrospray ionization mass spectrometry (ESI-MS) and electrospray tandem mass (ESI-MS/MS). The mass fragmentation patterns of this group of heterocyclic tetrahydro-beta-carboline compounds are described herein.