Mechanistic explanations for the downward excursion in d¹⁸O of atmospheric CO₂ observed during the mid-1990s and the generally large interannnual variability characteristic of this isotopologue are lacking. We hypothesize that the excursion and related variations in d¹⁸O of atmospheric CO₂ may be linked to global-scale variations in cloud cover. However, very little is known about the influence of clouds on biosphere-atmosphere CO¹⁸O exchanges. Recent work has demonstrated the influence of boundary layer clouds on canopy photosynthesis through increases in the diffuse radiation fraction and relative humidity, combined with decreases in leaf temperature. In concert, these alterations tend to increase canopy photosynthesis and conductance, which should also increase CO¹⁸O isofluxes. However, photosynthetic CO¹⁸O isofluxes also depend critically on the d¹⁸O of leafwater, and enhanced cloudiness typically decreases the d¹⁸O of leafwater by enhancing relative humidity and water vapor exchange across stomata. Thus, the net impact of differing cloud regimes on biosphere-atmosphere CO¹⁸O exchanges is difficult to predict. Preliminary simulations suggest a large impact of diffuse radiation on canopy photosynthesis by increasing the flux from shade leaves. The impact of this effect on biosphere-atmosphere CO¹⁸O exchanges is diluted somewhat by the lower enrichment in leafwater d¹⁸O on cloudy days with high diffuse radiation fractions. Our results suggest that these effects are very dependent on LAI and photosynthetic pathway (C₃ or C₄).