This study analyzed controls on evapotranspiration (E) from a western Siberian bog, from a perspective that surface constraints limit the E available for atmospheric evapotranspiration demands. Ratios of E to potential evapotranspiration (EP) ranged from 0.2 to 0.9, and were clearly related to changes in surface constraints represented by the bulk transfer coefficient for latent heat CE (= ¿CH where ¿ is the surface moisture availability and CH is the bulk transfer coefficient for sensible heat). Both EP and equilibrium evaporation (EEQ) showed similar seasonal trends, suggesting the importance of radiation to the seasonal variation of EP. The atmospheric drying power (Ea) was a minor factor in EP and showed less seasonal change during most of the growing season. However, the presence of a dry air mass caused by synoptic scale advection (most frequently observed in May 1999) significantly enhanced Ea; consequently, the seasonal maximum of EP occurred earlier than the seasonal maximum of EEQ. Values for CH (0.004--0.011) increased with leaf area index except Sphagnum moss (LAIg), indicating that vegetation growth contributes to changes in bog roughness through canopy height changes. The ¿ value gradually decreased with decreases in the water table position (zwt); the open water surface area and water content of Sphagnum moss depended on zwt. Furthermore, the absence of a significant relationship between ¿ and phenology implies that changes in evaporation contribute to variations in E more than changes in transpiration. Hence roughness change created by vegetation growth and surface wetness limit evapotranspiration to less than the potential evapotranspiration.