The role of residual lithium carbonate in the electrochemistry and outgassing of lithium transition-metal oxides (TMOs) has been largely overlooked. By combining in situ gas analysis, isotopic labeling, and a surface carbonate titration, we show that the presence of residual lithium carbonate (Li2CO3) on the surface of both Ni-rich Li-stoichiometric (specifically LiNi0.6Mn0.2Co0.2O2) and Li-rich (Li1.2Ni0.15Co0.1Mn0.55O2) TMOs has a direct correlation with the amount of CO2 and CO evolved and has a relationship with O-2 evolved from the TMO lattice on the first charge. By selectively isotopically labeling the residual surface Li2CO3, which remains in small quantities (similar to 0.1 wt %) after synthesis, and not the carbonate electrolyte, we further show that, up to 4.8 V vs Li/Li+ on the first charge, carbonate electrolyte degradation negligibly contributes to gas evolution. These key conclusions warrant a reassessment of our notion of oxidative decomposition of carbonate electrolytes on TMO surfaces and, more generally, the reactivity of TMO surfaces. For the battery research community, our results highlight the importance of quantification of the surface contaminants and suggest that further research is needed to fully understand the long-term effects of trace surface Li2CO3.