Classification of 2000 food aroma compounds as to their molecular weight shows an accumulation in the range of 135 to 155, and an upper limit of 310. Molecular masses of substances with high aroma effectiveness are below 200. As aroma compounds are composed only of the elements C, H, O, N and S, even numbers of molecular masses predominate among these substances. A periodicity with an interval of 7 units of molecular mass observed seems to be caused by the presence of complete homologuous series of compounds within the range of low molecular masses.