In order to develop a bioreactor for solid to solid conversions the biocatalytic conversion of solid Ca-maleate to solid Ca-D-malate is studied. The dissolution of Ca-maleate is the first step in this process and is described here. A kinetic model, based on the interfacial-barrier theory and the diffusion-layer theory, was developed which describes the increase in Ca-maleate concentration due to dissolution with the help of the time-dependent parameters. According to the model two processes contribute to the dissolution of Ca-maleate . H2O crystals : (1) the dissolution land dissociation) reaction of Ca-maleate at the solid-liquid interface, characterized by a time-independent reaction rate coefficient, and (2) the transport of Ca2+ and maleate(2-) across a boundary liquid film, characterized by a time-dependent mass-transfer rate coefficient. In addition, the surface of a crystal and the driving force are time;dependent variables. Since Ca-maleate . H2O crystals are not uniform, a crystal-size distribution was also used in the model. The effects of stirring speed, temperature, pH, and initial Ca2+ concentration on the dissolution rate of Ca-maleate . H2O crystals were determined experimentally in order to evaluate the model. The model fitted the data well (R-2 > 0.97). In order to determine whether the overall dissolution process was reaction or transport controlled, a method based on overall reaction and transport rates (per unit of driving force) was developed. This showed that the dissolution of Ca-maleate was reaction controlled. Temperature influenced the reaction rate coefficient the most; it ranged from 5.7 x 10(-6) m s(-1) at 10 degrees C to 67 x 10(-6) m s(-1) at 60 degrees C. The reaction rate coefficient was also influenced by the pH and the initial Ca2+ concentration, but, as expected, hardly by the stirring speed. Simplifying the model by omitting the time-dependent mass-transfer rate coefficient and by assuming uniform crystals, resulted in only slightly worse fits of the data with R-2 being at most 0.004 smaller.