The thermal decomposition of cobalt complexes of the types, Na[Co(HY)X]· nH 2O, H[Co(HY)X]· nH 2O, Na 2[CO(Y)X]· nH 2O, Na(Co(YOH)X]·H 2O, Na[Co(Y)]·3H 2O, and H 2[Co(Y)]·3H 2O, where Y is EDTA and YOH is HEDTA, X is Cl −, Br −, NO 2 − and OH −, and n may be 1–3, was studied by the techniques of TGA and DTA. The H[Co(HY)X]· nH 2O and Na[Co(HY)x]·H 2O complexes, when heated, first lost water of hydration in the 50–75° temperature range. Breaks in the TGA curve revealed that the water came off in two steps. The anhydrous complexes began to decompose in the 215–250°C temperature region. The halo-complexes, Na 2[ Co(Y)X]· nH 2O, decomposed by a different mechanism in which all of the water plus a carbon dioxide, the latter presumably from a decarboxylation reaction, were evolved in a one step process. The DTA and TGA curves of all of the complexes are discussed.