A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO2 reduction (CO2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO2R. On Ni-MoS2, a limitation is the kinetic barrier for the proton electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO2R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.