Heterogeneous U$_{1-x}$Th$_x$O$_2$ polished pellets were prepared through sintering of oxides obtained from hydroxide precursors. Mixing several homogeneous U$_{1-x}$Th$_x$O$_2$ powders with various stoichiometries allowed the preparation of a panel of heterogeneous sintered pellets with a matrix-inclusions type microstructure. The characterization of the polished pellets was performed by X-EDS. From image analysis of large cartographies of the pellets, the distribution and amount of each type of heterogeneity was evaluated. To follow the chemical durability of the prepared materials, the pellets were submitted to dissolution tests in 2 and 4 mol.L$^{-1}$ HNO$_3$ at 60 and 90 °C. The obtained dissolution rates of the materials highlighted the need to access a more detailed microscopic study due to the presence of enriched dissolution residues at the end of the experiments. The heterogeneities were simulated by using powders heated at 1600 °C to isolate their specific dissolution behaviour. The sintered powders surrogates dissolved extremely slowly in solution. However, the normalized dissolution rates increased when HNO$_2$ catalytic species were introduced in the solution. Another set of experiments obtained by varying the incorporation rates in heterogeneities showed a decrease of the dissolution rate with increasing incorporation rate of Th-enriched heterogeneities. This effect is counterintuitive as Th-enriched heterogeneities are not supposed to dissolve during the first instants of the dissolution. Microscopic study based on operando monitoring of the solid/liquid interface highlighted preferential dissolution zones with a dissolution front in the matrix and the refractory character of the heterogeneities. Finally, the collected residues of dissolution were also characterized. They showed slight dissolution progress with the formation of dissolution pits, preferential attacks of the grain boundaries and a mechanism resembling the cracking core model of dissolution. This work offers a first study of the dissolution of heterogeneous U$_{1-x}$Th$_x$O$_2$ mixed oxides with a matrix-inclusion type microstructure.