The reaction rate of the trimerisation of anhydrous formaldehyde vapour was examined in a Berty-type reactor at a temperature of 110 degrees C in the pressure range from 0.6 to 1.1 bar. 1-Vanado-11-molybdophosphoric acid on silicon carbide support was used as catalyst. The reaction rate shows a step-wise increase with growing partial pressure of formaldehyde. The reaction takes place in the pseudoliquid bulk phase of the catalyst. Intracrystalline diffusion within the heteropoly acid particles obviously is the rate-determining step of the reaction. Catalyst crystals do incorporate trioxane molecules during the reaction yielding H4PVMo11O40.nC(3)H(6)O(3) phases of well-defined compositions. These phases differ in their diffusion coefficients causing the exceptional shape of trimerisation kinetics.