In this study, we examine the magnitude and temporal variability of surface water pCO2 in a set of lakes in boreal Québec, and explore the links between lake and catchment properties. The study lakes were consistently supersaturated in CO2, with the mean lake pCO2 ranging from 400 to over 1800 μatm. There was significant interannual variability in pCO2, apparently driven by regional patterns in precipitation. The best multivariate model of average pCO2 included dissolved organic carbon (DOC), lake area and chlorophyll as independent variables, suggesting that external carbon (C) loading to lakes plays a central role in lake CO2 dynamics and that lake trophic status may modulate the influence of external C loading. We show that even if the key drivers of lake pCO2 are similar, they interact differently among regions and the resulting models may be dramatically different. In particular, we show that although pCO2 is invariably correlated to DOC, the shape of this relationship varies greatly among regions, suggesting large-scale regional differences in C delivery, quality, and in-lake processing. As a consequence, current models cannot be extrapolated across regions unless we apply region-specific variables.