The heats of successive protonation of 3,3′-diaminodipropylamine (dpt) have been measured calorimetrically at 25° in 0·1M-potassium chloride and all the values were found to be higher than those for 2,2′-diaminodiethylamine (den). The heats of formation of the complexes [Co dpt]²⁺, [Ni dpt]²⁺, [Ni dpt₂]²⁺, [Cu dpt]²⁺, [Zn dpt]²⁺ and of the hydroxo-complexes [Cu dpt(OH)]⁺ and [Zn dpt(OH)]⁺ were also measured under the same conditions and all found lower than those of the corresponding den complexes. By combination of the values of the enthalpy changes with the corresponding free-energy changes, reported elsewhere, values of the entropy of protonation and of complex formation have been calculated. The lower stability of the six-membered chelate rings was, except for the cobalt(II) complex, due to their lower heat of formation. The enthalpy contribution increases with the stability of the complex, while the entropy contribution decreases linearly from cobalt(II) to zinc(II). The infrared spectrum of [Ni dpt₂](ClO₄)₂ does not show frequencies to be attributed to unco-ordinated amine groups. The visible spectra of the coloured ions can be interpreted in terms of octahedral structures with various degrees of distortion. The zinc(II) complexes, however, appear to have a tetrahedral configuration. The entropy of formation of the hydroxo-complexes is the same and is independent of the metal ion.