CO2 and hydrocarbon fluxes from a sitka spruce forest were measured using a conditional sampling method. The method was used in two ways: (1) an automated system was used to monitor continuously the mixing ratio difference of total hydrocarbons and CO2 in sampling lines for updraught and downdraught air and (2) conditionally sampled updraught and downdraught air was passed through adsorption tubes, which subsequently were analyzed in the laboratory, to determine the fluxes of nonmethane hydrocarbons (NMHCs). Aerodynamic gradient measurements of NMHC fluxes were made over the same period. Method (1) produced a high temporal resolution data set of approximately 4 days of near-continuous fluxes. Marked diurnal trends in CO2 flux were evident. Peak daylight photosynthetic fluxes ranged from −30 to −90 mmol m−2 h−1. Smaller fluxes were noted during heavy rainfall and diminished photosynthetic ally active radiation. Nighttime respiration ranged from 0 to 10 mmol m−2 h−l. Total hydrocarbon fluxes were 3 orders of magnitude smaller. During hot, sunny conditions a total hydrocarbon flux of approximately 400 μmol m−2 h−1 (CH4 equivalents) was observed. Fluxes of specific NMHC compounds, from method (2), ranged between 90 and 563 μg m−2 hour−1. The annual carbon flux to Great Britain's conifer forests was estimated to be 1.3 Mt using a simple upscaling model based on the observations of CO2 flux during the field experiment.