Analyses of the relationship between cloudiness and net ecosystem exchange of CO₂ (NEE) are presented for two AmeriFlux forest sites (Morgan-Monroe State Forest, south-central Indiana and University of Michigan Biological Station, northern lower Michigan), which vary in latitude and species composition. Cloud cover affects the ratio of diffuse to direct incoming radiation, and thus influences the availability of photosynthetically active radiation (PAR) through the depth of the canopy. The methods of Gu et al. (1999) are followed with the exception that a cloudiness index is derived from the difference between observed hourly insolation and that modeled for the same hour but using clear sky conditions, with particular attention to optical transmissivity in the absence of clouds. The role of cloudiness on NEE is considered in the context of observed levels of PAR for the two forests, as well as other governing environmental variables, including soil moisture and temperature, vapor pressure deficit, air temperature and vegetation area index. The importance of potential long-term change or interannual variability in cloud cover for enhancing or reducing NEE at these sites is also assessed.