Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) was used to determine the stability constants of the actinides Am(III), Th(IV), Np(V), and U(VI) at an ionic strength of I = 0.3 M. The obtained stability constants were extrapolated to zero ionic strength by means of the Davies equation. For both U(VI) and Am(III), three consecutive acetate complexes with log(beta(0)(1)) = 3.01 +/- 0.12, log(beta(0)(2)) = 5.27 +/- 0.07, log(beta(0)(3)) = 6.82 +/- 0.09, and log(beta(0)(1)) = 3.70 +/- 0.09, log(beta(0)(2)) = 5.35 +/- 0.08, log(beta(0)(3)) = 6.45 +/- 0.09, respectively, could be identified. For Np(V), there was just one acetate complex, with log(beta(0)(1)) = 1.56 +/- 0.03. In the case of Th(IV), five different complex species could be determined: log(beta(0)(1)) = 4.73 +/- 0.16, log(beta(0)(2)) = 8.92 +/- 0.09, log(beta(0)(3)) = 12.16 +/- 0.11, log(beta(0)(4)) = 12.96 +/- 0.87, and log(beta(0)(5)) = 14.39 +/- 0.16. The actinides were selected with regard to their most stable oxidation state in aqueous solution so that four different oxidation states from +III to +VI could be investigated. A great benefit of CE-ICP-MS is the opportunity to measure at significantly lower concentrations compared to the available literature, allowing the study of actinide complexation in environmentally relevant concentration ranges. Furthermore, it is possible to analyze all four actinides simultaneously in one and the same sample.