Canopy level flux measurements of 2-methyl-3-buten-2-ol (MBO) were made over a 7 year old ponderosa pine plantation in the Sierra Nevada range of California using the relaxed eddy accumulation method. Fluxes peaked in late afternoon (1600 LT), at an average rate of 1.11 mg C m-2 h-1. This corresponds to the time of peak temperatures. MBO fluxes were strongly correlated with both temperature and photosynthetically active radiation (PAR). Model calculations of MBO flux from this site using a model developed for isoprene provide an accurate simulation of the diurnal emissions pattern but overestimate the measured flux by a factor of 2. Discrepancies between the measurements and the model predictions are likely due to the uncertainties in choosing model inputs. These results indicate a significant flux of reactive carbon in the western United States where MBO-emitting pines are the dominant forest vegetation. ¿ 1999 American Geophysical Union