Abstract: An apparatus for measuring components of liquid samples, such as milk samples, based on the ability of these components to absorb infrared radiation at certain frequencies. A single radiation beam is directed through a cuvette containing the sample, and the apparatus comprises a plurality of pairs of optical filters. Each of these pairs is associated with a respective one of the components to be measured and comprises a component filter for passing infrared radiation at a narrow frequency band at which the radiation absorbing ability of the said component is relatively high, and a reference filter for passing infrared radiation at a different narrow frequency band at which the radiation absorbing ability of the component is lower.