Abstract: Fat in a fat-containing sample, in particular a milk sample, is determined quantitatively by infrared absorption technique, using as sample waveband a band characteristic to carbon-hydrogen bonds, instead of the conventional triglyceride carbonyl band. This avoids the dependency of the measured result on the fat composition of the sample. Influences from other components in the sample are compensated for. A preferred method uses double wavelength determination, with sample wavelength in the interval from 3.475 to 3.51 .mu.m and reference wavelength in the range between 3.51 and 3.60 .mu.m.