Gas phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene has been studied by using silica-supported heteropolyacids. The activity of a Dawson-type tungstophosphoric acid, H6P2W18O62/SiO2, was comparable to that of a sulfonated polymer resin, Amberlyst 15, and the selectivity was higher than that of Amberlyst 15. A Keggin-type heteropolyacid, H3PW12O40/SiO2, was also efficient for this reaction, while unsupported H3PW12O40 was much less active than unsupported H6P2W18O62. Calorimetry of NH3 absorption revealed that the difference in acid strength between H6P2W18O62 and H3PW12O40 was large, but the difference between the two became smaller when they were supported. This partly explains the similar activities of the two supported heteropolyacids. By supporting H6P2W18O62 On SiO2, the MTBE selectivity on the basis of methanol increased, but the selectivity on the basis of isobutylene decreased, which was reasonably explained by the decrease of the relative contribution of pseudoliquid phase reaction to the surface reaction by supporting. The pressure dependency on methanol for H6P2W18O62/SiO2 was close to that of unsupported H6P2W18O62, but it much differed from those of ordinary solid acids like SO42-/ZrO2 and H-ZSM-5, showing that MTBE synthesis proceeds in the pseudoliquid phase of H6P2W18O62/SiO2.