Since the mobilisation of radioelements in final repositories is strongly dependent from the environmental conditions, an investigation of the effect of ions present in groundwater on the mobilisation processes was performed. UO2 was leached in a well-characterized groundwater under ambient temperature / pressure and anoxic redox conditions. The content of the ions calcium, carbonate and silicate was enhanced gradually in three steps. The presence of silicate suppressed significantly the solubility of uranium. Secondary phases in a beginning state of formation were observed on the sample surfaces. Surface examinations were performed using optical microscopy and SEM-EDAX. Thermodynamic calculations (geochemical modelling) using the code PHREEQC (Version 2.3) were performed to simulate the behaviour of the systems. Uranium concentrations in solutions as well as the precipitation of several solid products were predicted. The values, however, differ from the experimental results. The modelling took into account the redox environments.