The nature of the intermolecular force field of the nitriles is considered on the basis of the electron orbital structure and charge distribution of the nitrile group. The directional nature of the force field is due to a well-directed lone pair orbital on the N atom, which may be expected to exhibit strong donor properties, and two π-orbitals which may exhibit weak donor properties. Accordingly with good acceptor molecules such as chloroform and hydrogen chloride, simple 1:1 molecular addition compounds should occur. The existence of molecular complexes of this type was confirmed with the aid of binary freezing-point diagrams which were determined for aceto-, propio-, butyro-, and benzo-nitrile with chloroform and hydrogen chloride. The 1:1 association complex was absent, however, in the system acetonitrile–chloroform. This is accounted for by the stronger association occurring in acetonitrile itself, the nature of which is discussed. The structure of the 1:1 molecular complexes is considered. Additional molecular complexes with lower nitrile mole ratios are indicated in the freezing-point diagrams. Of particular interest are the well-defined compounds appearing in the nitrile – hydrogen chloride systems with the composition RCN•5HCl. The possibility that the π-orbitals of the nitrile group may function as donors in these compounds is discussed, and a tentative structure is suggested.