SUMMARYA model is presented of the turbulent mixing between a sphere of droplet-free air (saturated or undersaturated) and a spherical volume of cloud within which it is embedded. The rates of equalization of supersaturation, temperature and liquid water are examined as functions of the turbulent energy dissipation rate, the dimensions of the interacting volumes, the drop-size distribution in the cloud and other parameters. Spectral changes resulting from the mixing conform closely with those predicted by the recent inhomogeneous or dilution-only descriptions of mixing (Baker et ol. 1980; Telford and Chai 1980).It is shown that spectral changes observed during entrainment into shallow cumulus, stratocumulus and cap clouds are in good agreement with those predicted on the basis of this diffusive model.Calculations of mixing in the presence of an updraught yield size-distributions (including bimodal spectra) similar to those observed in natural clouds.Calculations of mixing during descent from cloud-top suggest that regions of substantial undersaturation may exist in the deep interiors of clouds which are entraining environmental air at their summits. Crude estimates of the degree of lateral spreading produced by turbulence as this descent proceeds indicates that significant horizontal structure in the water properties may exist throughout a substantial fraction of the cloud-depth.