SUMMARYThe effect of fat, pH, and volatile acids on the perception of diacetyl was investigated. At the normal pH of skim milk or cream the threshold for diacetyl was 0.01 ppm. At pH 5.0 the threshold in skim milk was 0.2 ppm, and at pH 4.4 the diacetyl threshold in cream was 0.05 ppm. The fat content of cream probably suppresses the acid effect. The presence of propionic acid did not lower the threshold of diacetyl in sour cream. Formic acid lowered the threshold slightly, whereas acetic acid and acetaldehyde lowered it considerably.