Nuclear magnetic resonance measurements for the system chloroform+ pyridine, in the temperature range 21-7-6O.O0C, have been used to estimate the standard enthalpy of association as A H = -101 kJ mole-1. Calorimetric measurements of the heat of mixing have been made at 25°C for binary mixtures of chloroform with pyridine and with cyclohexane. Attempts have been made to relate the spectroscopic and calorimetric results.As part of a study of the thermodynamic properties of solutions in which hydrogen bonds are present, the association of pyridine with a series of alcohols 1 has been investigated. In such a solution, however, there are two competing equilibria : the association of an alcohol molecule with a pyridine molecule to form a complex, and the self-association of the alcohol molecules to form dimers or higher polymers. If one is interested in determining the equilibrium constant for the former reaction, then one has to make allowances for the self-association. In an attempt to find two components which could form a complex, but which would self-associate to a negligible extent, it was decided to study chloroform with pyridine. Although the C-H bond will not normally take part in hydrogen bonding, it will do so in a molecule like chloroform. Chloroform molecules do self-associate,2* 3 but the equilibrium constant for this is much less than that for an alcohol.This paper describes two approaches to the problem of determining the hydrogen bond energy between chloroform and pyridine. The first is spectroscopic and the second calorimetric. Since this work was commenced, Berkeley and Hanna have published4 their n.m.r. results for the same system, but over a lower and more restricted temperature range. EXPERIMENTAL MATERIALSBenzene, carbon tetrachloride, chloroform and pyridine were purified by standard procedures, in which the drying agents were phosphorus pentoxide for the first three and sodium hydroxide 7 for the last.