Abstract: The catalytic behavior in bulk of the Keggin type heteropoly acid H₃PW₁₀O₄₀(abbreviated as HPA) has been investigated by varying amount of HPA on SiO₂With the catalysts containing HPA of 5×10^-5-1.5×10^-1mol/(dm)³of SiO₃it was found that the acid site on the surface and in the bulk of HPAshowed the same turnover frequency for the dehydration of iso-propanol. But the selectivity of catalysis in the bulk differed from that on the surface of HPA. Catalysis in the bulk favoured the formation of products with more than four carbon atoms in methanol conversion and the formation of ethylene in dehydration of ethylalcohol.