Experimental studies on the design of shell- and tube-type heteropolyacid (HPA)-polymer composite catalytic membrane reactors (CMR) and their applications to the vapor-phase decomposition of methyl tent-butyl ether (MTBE) were reported. H3PW12O40 (PW) was used as a catalyst for the reaction, and polyphenylene oxide (PPO) was used as a membrane material. It was found that the single-phase (PW-PPO/Al2O3, type-1) and the composite (PW-PPO/PPO/Al2O3, type-2) catalytic membranes not only showed catalytic reactivity for the reaction, but also were perm-selective for methanol with respect to isobutene and MTBE. The selective removal of methanol through the catalytic membrane inhibited the unfavorable reverse reaction (MTBE synthesis), and at the same time accelerated the MTBE decomposition reaction. The PW-PPO/PPO/Al2O3 CMR showed a superior performance compared to the PW-PPO/Al2O3 CMR. The enhanced performance of PW-PPO/PPO/Al2O3 CMR was attributed to the intrinsic perm-selective capabilities of the PW-PPO catalytic membrane and the sub-layered PPO membrane. The PW-PPO/PPO/Al2O3 catalytic membrane with high PW loading was less perm-selective than that with low level of PW loading. Although the PW-PPO catalytic layer itself had a lower intrinsic activity than the bulk PW catalyst in a normal reactor, PW-PPO/PPO/Al2O3 catalytic membrane could be utilized in a membrane reactor to achieve higher MTBE conversions than equilibrium values by means of chemical equilibrium shift at high cut-off.