The limiting mechanisms in dimethyl ether steam reforming (DME SR) are experimentally investigated over the nanocomposne catalysts of Cu spinel and alumina. The dominant reactions involved in DME SR are proved lobe DME hydrolysis to methanol over acidic sites of alumina and subsequent methanol steam reforming to H-2 and CO2 over Cu sites, followed by reverse-water gas shift reaction The contribution of other side reactions is also clearly evaluated DME hydrolysis is limited by the equilibrium, and the hydrolysis rate was much slower than the methanol SR rate. and thus was a rate-determining step in DME SR The deactivation of Cu spinel was significantly faster than that of alumina, and would determine the lifetime of the composite catalysts With varied superficial velocities up to 5 2 cm s(-1) (25 degrees C, 1 atm). the evidence of mass transfer-limiting mechanism and reaction-limiting mechanism was observed in the low flow rate and the high flow rate regions. respectively (C) 2010 Elseviei B V All rights reserved