The conversion of sodium carbonate to sodium carbonate monohydrate and the subsequent growth of monohydrate crystals have been investigated by carrying out in situ measurements of particle size distribution by means of the focused beam reflectance method (FBRM). The conversion of sodium carbonate to sodium carbonate monohydrate is found to be a solution-mediated transformation. The formation of monohydrate crystals is explained by a supersaturation-nucleation-growth mechanism. Growth rate calculations show that monohydrate crystals grow at higher rates at lower aqueous phase temperatures, while induction period measurements suggest that sodium carbonate monohydrate crystals nucleate more rapidly at lower temperatures. Both effects can be described by allowing that during the process the solution is saturated with respect to the dissolving sodium carbonate and supersaturated with respect to the growing sodium carbonate monohydrate. As temperature decreases, the supersaturation increases.