Diacetyl content of cow butter is one of the factors determining its quality. It is a metabolic product of aroma producing lactic acid streptococci. The gradation of diacetyl water solutions and the correlation between diacetyl quantity in cow butter and its taste, flavour and aroma evaluation were assessed by smelling. It was confirmed that experienced degustators of dairy butter can distinguish and grade diacetyl water solutions in a comparatively very good order. A satisfactory correlation was observed between the organoleptic evaluation of the taste, flavour and aroma of cow sour cream butter. In the opinion of experienced degustators of cow butter the latter can be graded only organoleptically.