We present a comprehensive mechanistic study on the highly tunable selectivity over In x /ZrO2 catalysts in CO2 hydrogenation. By variation of the indium loading between 0.1 and 5 wt %, either an admirable selectivity to methanol of 70–80% or up to 80% selectivity to CO could be obtained in the temperature range of 250–280 °C. It is shown that the shift in the product spectrum is related to the synergy between indium species and the zirconia substrate through variable interfacial structures. Zirconia-modulated crystalline In2O3, which prevails for indium loadings between 2.5 and 5 wt %, could enhance stepwise hydrogenation of *HCOO, leading to *H3CO and finally methanol due to the suitable bonding strengths of *HCOO and *H3CO. Regarding CO, evidence has been provided that the synergistic effect between adjacent indium and zirconia sites is indispensable for the entire catalytic cycle. *HCOO is formed at the indium–zirconia interfaces and decomposes to CO subsequently. Highly dispersed InO x dominating for loadings below 0.5 wt % features an enormous indium–zirconia interface and suppresses hydrogenation ability for *HCOO, thus favoring the generation of CO. The study provides fundamental insights into the mechanism of CO2 conversion and reaction pathway tuning over oxide catalytic systems.