Sulfur dioxide oxidation on vanadium catalysts in sulfuric acid production may cause phase transitions in the catalyst active component. These phase transitions involve crystallization of vanadium(IV), and may influence catalyst activity. In the present study, we show that reaction mixture flow through a fixed catalyst bed is accompanied by the moving front of crystal phase, which decreases SO2 conversion at reactor outlet. Front profile and its movement velocity depend on the chemical composition of active component, support porous structure, catalyst particle size, and other parameters. Based on the Gibbs-Volmer theory, we suggest a simplified mathematical model, which (qualitatively and quantitatively) describes experimentally observed dynamics of front generation and movement, as well as related changes in the bed activity. The model allows us to predict catalyst behavior in an industrial scale reactor, and to optimize catalyst properties.