The δ13C of slash pine foliage dark-respired CO2 (δ13CF) was measured at temperatures ranging from 6.6¿ to 30¿C. The δ13CF was 13C-enriched initially after dark by as much as 9? and declined toward an asymptotic value after a few hours in the dark. Maximum enrichment in δ13CF occurred at the lowest temperatures and the decline in δ13CF toward the asymptotic value took longer at lower temperatures. The effect of this temporal variation in foliage enriched CO2 on determination of the δ13CO2 of ecosystem respiration (δ13CR) was investigated using two approaches by measuring the nighttime CO2 concentration and δ13CO2 ratio in a 30-m-tall pine forest in Florida, United States. The first approach (time series) consisted of making measurements at a single height at midcanopy from 1 hour after sunset until 0200 local time. The second approach (profile) consisted of making measurements at multiple heights from ground level to the top of the canopy synoptically prior to dawn. For each method a Keeling plot was used to estimate the δ13CR. The δ13CR values determined with the time series were 13C-enriched relative to those determined from profiles on average by 1.0? and by as much as 3.1?. A mass balance was used to determine the impact of the nonconstancy of δCF on the overall Keeling plot intercepts. For the months examined, results indicate that (1) the enrichment in the Keeling intercepts determined from time series compared to those determined from profiles was consistent with the addition of nighttime foliage respired 13C-enriched CO2 and (2) Keeling plots determined from midcanopy heights appear linear despite the addition of temporally variable source δ13CO2 (range: 4?). A common approach that specifies the time (predawn) and spatial distribution (multiple heights) for CO2 collection is suggested to improve intercomparisons of δ13CR from different ecosystems.