The results of two independent calorimetric investigations of the pyridine- iodine complex are reported. “Best” values are reported as Km = 128 1/mol and ΔH° = -8.4 kcal mol-1 for the formation of the complex in cyclohexane at 25°C, and Km = 104 and ΔH° = -7.9 kcal mol-1 in carbon tetrachloride. Evidence is presented to support the contention that association constants for weak complexes determined by calorimetric methods can be as reliable as those determined by spectrometric methods, and that values of ΔH° determined by the calorimetric method are much more reliable than those derived from the temperature dependence of equilibrium constants. © 1973.