We developed a tandem, unassisted, solar-driven electrochemical and photothermocatalytic process for the single-pass conversion of CO2 to butene using only simulated solar irradiation as the energetic input. The two-step process involves electrochemical CO2 reduction (CO2R) to ethylene followed by ethylene dimerization to butene. We assessed two unassisted electrochemical setups to concentrate ethylene in the CO2R reactor, achieving concentrations up to 5.4 vol% with 1.8% average solar-to-ethylene conversion and 5.6% average CO2-to-ethylene single-pass conversion under 1-sun illumina- tion. When the effluent electrochemical gas stream was passed through the photothermocatalytic ethylene oligomerization reactor, we generated 600 ppm of butene under 3-sun illumination. Through analysis of this process, we identified that the presence of H2, CO, and H2O leads to rapid deactivation of the Ni-based ethylene oligomerization catalyst.