Seasonal emissions of isoprene and other light hydrocarbons were measured from both an elevated dry site (hummock) and low wet site (flark) on a Sphagnum fen in Sweden, and diurnal emissions were measured from a wet site on a Sphagnum fen in Finland. Both fens are in the southern boreal zone. Samples were taken from a Teflon chamber with stainless steel canisters, and analysis was done by gas chromatography with flame ionization detection. Isoprene clearly dominated the flux of light hydrocarbons, while ethene, propene, and 2-butene, as well as traces of n-butane and propane, were also observed. The isoprene emission season extended from the spring thaw to the occurrence of ground frost in the fall. The highest fluxes of isoprene were observed from wetter sites (flarks) and reached up to 2000 μg C m-2 h-1, while the fluxes from the hummocks did not exceed 70 μg C m-2 h-1. The fluxes of ethene and propene did not exceed 15 μg C m-2 h-1. All emissions showed strong diurnal variations with zero or near-zero values at night. Much of the data can be explained by temperature and light, as is the case for isoprene-emitting trees, but other factors are important. The mean normalized emission rate for the wet Sphagnum environments was 624¿357 μg C m-2 h-1 (photosynthetic active radiation equal to 1000 μmol m-2 s-1, 30 ¿C). Our data indicate that in comparison with emissions from forests, the flux of isoprene from Sphagnum peatlands is a significant source of volatile organic compounds to the atmosphere in boreal regions. ¿ 1998 American Geophysical Union