The boiling point of a monofunctional organic compound is expressed as the sum of two parts: a contribution to the boiling point due to the R group and a contribution due to the functional group. The boiling point in absolute temperature of the corresponding RH hydrocarbon is chosen for the contribution to the boiling point of the R group and is a measure for the strength of the interchain interactions. The boiling point and molar refractivity increments between a monofunctional compound and its corresponding RH hydrocarbon are, together with the dipole moment of the monofunctional compound, the properties to describe the influence of the functional group on the boiling point and on the strength of the corresponding intermolecular forces. The ranking of the functional groups according to the strength of their interactions and the masking of these interactions by the R group are investigated. Mathematical equations are derived for the computation of the boiling point increments. The method is applied to the members of monofunctional homologous series and to phenyl derivatives of the functional groups. (Contains 4 tables.)