We present results from an experiment that was designed to investigate turbulent transport relationships in a nearly homogeneous boundary layer disturbed by unsteady wind swings, as found at thebase of an advective inversion with a convective boundary layer overhead. In such a situation we measured vertical gradients and eddy fluxes of temperature and humidity at two heights. From these, the turbulent diffusivities of heat and water vapour are obtained,and compared to the predictions of Monin–Obukhov similarity theory and those of a numerical second-order closure model. It is found that the measured diffusivities exceed both predictions. This is interpreted as a consequence of the unsteadyconditions. It is also found that the diffusivity forheat is roughly 10% larger than that for watervapour. This is in agreement with a theoreticaltreatment of the unsteadiness effects that we developed in an earlier publication. This result is not reproduced by the numerical model because the model has no provision for unsteady conditions. Our result disagrees with that from an earlier, very similar, field experiment, which may be due to a systematic underestimation of sensible heat flux in the older experiment.