The effect of instrument baseline averaging on the estimated spectrum density of a homogeneous random field of a scalar variable in turbulent flow is investigated. The spectrum transfer function of the spatial filter is evaluated for all orientations of the baseline to the vector mean wind. It is shown that the influence of baseline averaging on the spectrum increases as the angle between the baseline and the vector mean wind decreases and as the ratio of the baseline length to the wavelength of the fluctuations increases. These results can be used to correct the computed spectra of spatially averaged components of the velocity field in turbulent flow and of spatially averaged conservative passive additives in a turbulent flow.