The next generation application of nuclear energy has shown an interest towards ThO2 or mixed oxides fuels. However, more knowledge is needed, for example, on the microstructural and matrix dissolution behaviour of ThO2. The objective of this study was to examine surface properties of thick ThO2 pellets by non-destructive alpha analysis method accompanied with Monte Carlo simulations before and after a leaching experiment. This study has been conducted to investigate the contents, and surface layer thickness of crystalline 232ThO2 pellet leached in aqueous solution with excess of 229Th tracer in the beginning of the experiment. The simulation results have suggested a maximum 0.1 µm concentrated layer of 229Th on the pellet surface. The presence of this layer suppresses further dissolution of the pellet.