A comparison is made between two methods for determining the surface fluxes of sensible and latent heat during daytime. The first method, known as the Penman-Monteith approach, incorporates a more complete description of the physics. However, it needs a relatively large number of input parameters, which is inconvenient in many applications. The second method is a modification of the Priestley-Taylor evaporation model, which needs only net radiation, air temperature and an indication of the moisture condition at the surface. Both models are compared on the basis of hourly micro-meteorological data above short grass obtained in the Netherlands during the summer of 1977. The experiments were performed under predominantly unstable conditions [0 ≥ z/L0 ≥ −0.3z = (mean) measuring height, L0 = Obukhov length] with weak or no advection. It appears that, under these environmental conditions, the models have a similar skill. Therefore, the simple parameterization is preferred for practical purposes. It reveals that this result can be partially explained by the fact that the so-called equilibrium latent heat flux density LEEQ and vapor pressure deficit are correlated. The method requires further verification for different climatological conditions.