A simple explicit relation linking H-1 NMR chemical shifts and mixing enthalpy has been found for systems that form 1:1 complexes by hydrogen bonding. Starting from relatively general assumptions, we have derived two forms of this relation. The first form is represented by proportionality between the solution enthalpy of the proton donor and the chemical shift change of its active hydrogen atom. The second form is mediated by the relative extent of hydrogen bond formation, eta(1,rel), a quantity that can be directly evaluated from measurements of either the H-1 NMR chemical shift or the mixing enthalpy. The validity of both relation forms has been convincingly demonstrated for 12 systems formed by a strong proton donor (halothane [1-bromo-1-chloro-2,2,2-trifluoroethane] or trichloromethane) and an oxygen- or nitrogen-containing proton acceptor (aliphatic or alicyclic ether, piperidine).