The interaction of uranyl with acetate is studied by affinity capillary electrophoresis in aqueous acid solutions at the pH values 2.0 and 2.5. The use of data on metal ion mobilities at different pHs allows to establish the ligand species interacting with metal ion and complex species formed. The formation of two complex species UO2CH3COO+ and UO2(CH3COO)2 is observed (acetic acid concentration is up to 0.8 M). In the case of uranyl-acetic acid system, the viscosity of solution is significantly changed with an increase of acid concentration. For calculation of ion mobilities the viscosity changes are taken into account. The stability constants are calculated at the ionic strengths 0.02 and 0.05 mol L−1. The logarithms of the thermodynamic stability constants (β°) calculated with Davies equation for the activity coefficients of the ions are View the MathML sourcelog β1°=2.94±0.08 and View the MathML sourcelog β2°=5.50±0.15 at 25 °C. Obtained values are compared with literature data.