The kinetics of the trimerization of anhydrous formaldehyde vapour were examined in a tube reactor in the temperature range from 102.5 degrees C to 110 degrees C and in the pressure range from 0.7 to 1.15 bar, 1-Vanado-11-molybdophosphoric acid on a silicon carbide support was used as catalyst. Evaluation of the data obtained shows a steady increase of the reaction rate with growing partial pressure of formaldehyde. A description of the rate by a power-law leads to a formal reaction order of six. This high formal order could not be explained satisfactorily. Investigations in a Berty-type recycle-reactor showed that the reaction rate increased stepwise with growing partial pressure of formaldehyde. Intracrystalline diffusion within the pseudo-liquid phase of the heteropolyacid particles obviously is the rate-determining step of the reaction. The temperature dependence of the reaction rate was found to be inverse and extremely strong. Differences in the shapes of tube reactor and recycle reactor kinetic curves can be explained by the levelling effect of temperature gradients in the catalyst bed of the tube reactor.