The complexes NiL(2)X(2) . nH(2)O (L=N-1-isopropyl-2-methyl-1,2-propanediamine; X = 0.5SO(4)(2-) when n = 0; X = I- when n = 2; X = CF3CO2- when n= 0 or 2 and X = 0.5SeO(4)(2-) when n = 0 or 3) have been synthesized and their thermal behaviour has been studied. All the hydrated complexes are yellow in colour and on heating lose water of hydration accompained by a yellow --> light blue thermochromic transition, except in the case of the iodide complex, which retains the original yellow colour on dehydration. However, NiL(2)I(2) undergoes an irreversible exothermic phase transition (166-193 degrees C; Delta H = -7.5 kJ mol(-1)) with change in colour from yellow to reddish yellow. NIL(2)(CF3CO2)(2) shows a reversible phase transition(heating cycle, 144-170 degrees C; Delta H = 3.1 kJ mol(-1); cooling cycle, 158-135 degrees C; Delta H = -3.1 kJ mol(-1)) with no visual change in colour. All the yellow complexes possess square-planar geometry and the light blue complexes NIL(2)X(2)(X = CF3CO2-,0.5SO(4)(2-) and 0.5SeO(4)(2-)) are expected to be pentacoordinated. The phase transition occurring in NiL(2)X(2) (X = I- and CF3CO2-) is due to conformational changes of the diamine chelate rings.