Conformational changes in some nickel(II) diamine complexes have been studied by differential scanning calorimetry. Two tris(ethylenediamine)nickel(II) thiocyanates, (1) and (3), have been synthesised. Complex (1) undergoes a phase transition on heating, transforming into species (2)(at 416 K, ΔH= 1.7 kJ mol–1) which reverts to (1) upon cooling. Complex (3) does not undergo a phase transition. Conformational forms with a particular configuration about the metal ion, δδλ, δλλ, δδδ, have been suggested for complexes (1)–(3) respectively on the basis of the enthalpy change for (1)→(2) and the interconversion (1)→(3) in solution. The complex [Ni2(en)4(NCS)4](4), having both bridging and chelating ethylenediamine, has also been synthesised. On heating it undergoes a solid-state reaction (406–434 K, ΔH= 9.8 kJ mol–1) yielding trans-chelated monomeric [Ni(en)2-(NCS)2](5). The complex [Ni(pd)2(NCS)2](6)(pd = 1,3-propanediamine) has been found to transform into an isomer (7) after two overlapping phase transitions (446–467.5 K, ΔH= 20.1 kJ mol–1). Isomer (7), on keeping overnight in a humid atmosphere (relative humidity 60–70%), is transformed into another isomer (8) which on heating undergoes phase transitions in two steps (445–462 K, ΔH= 9.7 kJ mol–1; 465–477 K, ΔH= 3.9 kJ mol–1). The intermediate species (9) has been isolated. Conformational changes such as trans-chair-chair (6)→ skew boat-skew boat (7)→cis-chair-chair (8)→ skew boat-skew boat (9)→ species (10)[(7)] have been proposed on the basis of enthalpy changes for the interconversions. The complex [Ni(pd)3]Br2·H2O (11) after dehydration shows a phase change yielding an isomer (13)(408–428 K, ΔH= 7.1 kJ mol–1). Isomer (13) reverts to (11) on addition of one molecule of water. This phenomenon is probably due to a chair skew boat transformation of the chelated ligand.