The relationship between incident light intensity and net ecosystem CO₂ exchange (NEE) was examined at a low-shrub bog located near Ottawa, Canada. Shrub height was 0.25 m and maximum leaf area index was 1.3 at the peatland. Light intensity was expressed as a clearness index (CI), where values approaching zero indicate heavy overcast conditions and values approaching unity represent cloudless conditions. Light saturation with respect to net CO₂ uptake at the canopy scale occurred at low CI, near 0.3. Contrary to reports from some forest ecosystems, in which peak NEE occurred with CI in the range 0.4–0.7, at the peatland there were no differences in NEE across all ranges of CI above 0.3. At the same time, CI in the range of 0.4–0.7 was infrequent and of short duration relative to clear skies and thick overcast conditions. Finally, we show that the use of half-hourly average radiation measurements to determine CI can lead to significant overestimation of the CI index in the range between 0.4–0.7.