Measurements of a scalar flux from an extended surface are frequently made with the eddy correlation technique consisting of a vertical velocity sensor and a sensor for the scalar of interest. In many cases the two sensors have to be mounted with a significant horizontal separation to avoid flow interference. Consequently, the technique will underestimate the scalar flux. This paper addresses the issue of flux underestimation due to this separation. A model is developed in the framework of Monin-Obukhov similarity for the spatial covariance of vertical velocity and air temperature in the unstable surface layer. It allows the underestimation of sensible heat flux to be assessed using information on separation orientation relative to wind direction, atmospheric stability, measurement height, and separation distance. The coefficient in the model is evaluated with observations made over a potato field and a clover field. The principles established here should also be applicable to fluxes of scalars other than sensible heat. ¿ American Geophysical Union 1994