The maximum heats of mixing of chloroform with fifteen organophosphorus compounds were determined. Representative members of the families of types: RzP (O) OR, RP (O)(ORh,(RO) $ PO,(RO) zP (OjCI, ROP (O) Cl+(RO) aP and (RO) z-POH display differences in their hydrogen bonding ability, as reflected by the heat of mixing. These differences are qualitatively predictable from the structures of the compounds, showing enhanced activity ...