In recent years, the frequent appearance of phenethylamine designer drugs on the illicit drug market has been a matter of concern for all authorities involved. New phenethylamine drugs are being introduced because these compounds are not covered by existing legislation. Therefore, the new drugs cannot be considered illicit drugs until their names are officially recognized. This paper describes a method to screen for and quantify four 2,5-methylenedioxy-derivatives of 4-thio-phenethylamine (2C-T-series) in human plasma, using capillary electrophoresis coupled with electrospray ionisation-mass spectrometry (CE-ESI-MS). Prior to CE-MS analysis, a simple liquid extraction was used for sample cleanup. The method was validated according to international guidelines.