The thermal reactions of bis(dl-4-methyl-1,2-pentanediamine)nickel(II) complexes have been studied in the solid phase. A new example of square-planar (low spin) to octahedral (high spin) isomeric transformation was discovered in the chloride. The transformation proceeded endothermically (DELTAH = 3.50 kJ mol-1) and was accompanied by a reversible thermochromism from yellow to violet-blue on heating.