An experimental mixture plan was used in order to assess the composition effects on the catalytic activity of a ternary Cu-ZnO-Al2O3 catalyst for the water gas shift reaction (WGSR) in a constrained region of the mixture space centered in a defined catalyst composition (Cu/Zn/Al=40/45/15). The obtained interaction coefficients indicated that an important role is played by the Al component on the Zn containing, Cu rich catalyst. XRD, FTIR, XPS, TG/DTA and activity test data gave support to the statistical assumptions. The catalytic activity for the WGSR was ascribed to metallic copper sites obtained from calcined and further reduced, well interdispersed mixtures of hydroxycarbonates.