► Direct injection HPLC–MS/MS used for measuring artificial sweeteners (ASs) in water. ► Degradation kinetics of aspartame (ASP) at different pH was performed. ► Diketopiperazine (DKP), represents main transformation product of ASP in water. ► DKP present only in wastewater effluents at low frequencies. ► Determination of levels of AS in different environmental water samples. A HPLC–MS/MS method is presented for the simultaneous determination of frequently used artificial sweeteners (ASs) and the main metabolite of aspartame (ASP), diketopiperazine (DKP), in environmental water samples using the direct-injection (DI) technique, thereby achieving limits of quantification (LOQ) of 10ngL−1. For a reliable quantification of ASP pH should be adjusted to 4.3 to prevent formation of the metabolite. Acesulfame (ACE), saccharin (SAC), cyclamate (CYC) and sucralose (SUC) were ubiquitously found in water samples. Highest concentrations up to 61μgL−1 of ACE were found in wastewater effluents, followed by surface water with concentrations up to 7μgL−1, lakes up to 600ngL−1 and groundwater and tap water up to 70ngL−1. The metabolite DKP was only detected in wastewater up to 200ngL−1 and at low detection frequencies.