Lactose is a disaccharide that consists of galactose and glucose fragments bonded through a β-1→4 glycosidic linkage. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose fragment can be in either the α-pyranose form or the β-pyranose form, whereas the galactose fragment can only have the β-pyranose form: hence α-lactose and β-lactose refer to anomeric form of the glucopyranose ring alone.

As it gives free radicals by mechanochemistry, it is possible to use lactose to follow by ESR (electron spin resonance) the energy used during a milling process.

Its empirical formula is C12H22O11 and its molecular weight is 342.3.