Dolomitization is one of the most important diagenetic processes, but the reaction rate and time scale of dolomitization remain a topic of controversy. We conducted experiments in which the reaction of single calcite crystals with a Mg-rich fluid at 200 °C leads to the formation of a zoned reaction rim consisting of magnesite and, for intermediate times, Ca-rich dolomite. From detailed documentation of the evolution of the microtexture and chemical composition of the reaction rim, we infer a kinetic model for calcite replacement by Mg-carbonates. The chemical gradient for the structure-forming elements Mg and Ca in the reaction rim and the evolution of the rim thickness strongly indicate that the overall reaction rate is controlled by diffusive transport through the porous reaction rim. The composition of the product phases is kinetically controlled and records the local composition of the interfacial fluid without requiring oversaturation of the reservoir fluid. Reactive transport models on dolomitization processes assume that the rate of dolomitization depends on the rate of dolomite precipitation, which is contradictory to our experimental evidence. Modeling carbonate replacement in natural systems requires detailed knowledge on the evolution of the microstructure controlling the physicochemical transport properties of elements in the pore fluid.