Electrochemical CO2 reduction is of high interest for production of non-fossil fuels. The reactivity of eight Cu foams with substantial morphology differences was comprehensively investigated by analysis of product spectrum and electrochemical in-situ spectroscopies (XANES, EXAFS, XPS, Raman). This approach provided new insight in reactivity determinants: (1) Morphological details, (2) stable Cu oxide phases, and (3) *CO poisoning of H2-formation are not decisive. (4) The electrochemically active surface area (ECSA) determines reactivity trends. (5) Macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at CuCat surfaces (operando Raman spectroscopy). We propose: (6) H2 and CH4 formation are suppressed by macroscopic buffer alkalization, whereas CO and C2H4 formation still proceed via a largely pH-independent mechanism. (7) C2H4 is formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.