This paper presents a study on the evaporation of water from the surface in the area of the Lesser Pieniny Mts., determined on the basis of measurements at the climatic-lysimetric station in Jaworki. The evaporation of water from the surface is diversified in specific 10-day periods and months of the vegetation season, and the total for April-September was 334.1 mm. The quantity of evaporating water was lower than the measured reference evapotranspiration, assumed to be the evaporation of a well-watered, compact grassy sward, and lower than the reference evapotranspiration calculated in accordance with the Penman formula in French modification and the Penman-Monteith formula. In the area of the Lesser Pieniny Mts., the evaporation of water can be expressed as a function of the deficit of air humidity and – to a lesser extent – of air temperature. The seasonal plant coefficients k_c determined on the basis of evaporation from open water surface E_w can be used successfully to determine the evapotranspiration of mountain grassland.