This thesis is concerned with the development and application of novel theoretical and experimental methodologies to study crystal dissolution processes across multiple lengthscales. In particular, it presents a versatile in situ multimicroscopy approach, comprising atomic force microscopy (AFM), scanning ion-conductance microscopy (SICM), and optical microscopy (OM) that is readily combined with finite element method (FEM) simulations. The methodology permits the quantitative 3D visualization of microcrystal morphology during dissolution with well-defined, high mass transport rates, enabling both the measurement of face-dependent dissolution rates and the elucidation of the dissolution mechanism. The approach also allows the determination of interfacial concentrations and concentration gradients, as well as the separation of kinetic and mass transport limiting regimes. The high resolving power and versatility of this new methodology is demonstrated on four different crystalline compounds with very different characteristics.

First, the dissolution kinetics of individual faces of single furosemide microcrystals are investigated by OM-SICM and FEM modeling. It is found that the (001) face is strongly influenced by surface kinetics, while the (010) and (101) faces are dominated by mass transport. Dissolution rates are shown to vary greatly between crystals, with a strong dependence on crystal morphology and surface properties.

A similar approach is then used to investigate changes in both crystal morphology and surface processes during the dissolution of bicalutamide single crystals, achieving high resolution with in situ AFM. It is shown that dissolution involves roughening and pit formation on all dissolving surfaces, and that this has a strong influence on the overall dissolution rate. FEM simulations determine that mass transport contributions increase as dissolution proceeds due to a continuous increase of the intrinsic dissolution rate constant, promoted by the exposure of high index microfacets.

The methodology is further developed to show that kinetic data obtained from OMSICM and AFM, which provide differing measures of kinetic parameters, are in good agreement when the different mass transport regimes of the two experimental configurations are accounted for. The robustness of the methodology is verified via studies of L-cystine crystals, while also providing insights into the dissolution mechanism by visualizing hexagonal spirals descending along screw dislocations.

Finally, the ability of the methodology to characterize processes with fast surface kinetics is demonstrated by the study of the proton-promoted dissolution of calcite single crystals. The approach allows the accurate determination of the near-interface concentration of all species during dissolution, as well as the intrinsic dissolution rate constant of the {104} faces, showing that surface kinetics play an important role in the dissolution process. Overall, this methodology provides a significant advance in the analysis and understanding of dissolution processes at a single crystal level, revealing the intrinsic properties of crystal faces and providing a powerful platform from which future studies can be developed.