Cumene cracking on supported H4SiW12O40 was studied in the temperature range 150-244 degrees C using a constant how microreactor. In the initial periods of runs, besides the two predominant products benzene and propylene some less important amounts of C-4, C-5, and C-6 hydrocarbons as well as toluene and ethylbenzene were obtained. Based on these results and separate experiments with propylene and benzene, a reaction pattern was proposed. The side products vanished after some 10-20 min of run time during which there was rapid deactivation of the catalyst and a decrease from an initially high conversion to 20-30% conversion depending on temperature. From the mass balance the amount of carbonaceous deposit was estimated and it was shown that the precursors for its formation are propylene and cumene. Arrhenius plots of the rate constants at a given coking degree indicate that below 170 degrees C at low coking degree it is the surface reaction which is the rate controlling factor. Above this temperature the reaction is predominantly a diffusion controlled process.