Understanding the heat balance structure of the surface of a marshy meadow and the dynamics of its changes were the objectives of observations carried out in the valley of the Biebrza River. A simplified eddy covariance system allowed measurements of the density of four main energy balance components: latent (LE) and sensible (S) heat fluxes, soil heat flux (G), and net radiation (Rn). The mean half-hourly values of the net radiation during our observations ranged from -51 W·m⁻² (nighttime) to 309 W·m⁻² (daytime). The amount of available energy was used for evaporation in the first place – the latent heat flux assumed the highest values in the outgoing portion of the heat balance throughout the entire data series. Mean half-hourly values of latent heat varied between -90 W·m⁻² (at 3. p.m.) and 194 W·m⁻² (at 1 a.m.). The values of sensible heat flux density were relatively low and ranged from -16 W·m⁻² to 96 W·m⁻². During the day, the soil heat flux density ranged from 2 W·m⁻² to 36 W·m⁻², and the flux was most often directed from the active surface into the soil profile. At night, it assumed values within the range -24 to -5 W·m⁻².