The synthesis and thermal analysis studies of several hydroxobridged homo and hetero trinuclear cobalt(III) complexes are reported. The complexes are of the type [M(H2O)x{(OH)2Co(en)2}2](SO4)2nH2O and [M(H2O)x{(OH)2Co(NH3)4}2](SO4)2nH2O where en denotes ethylenediamine and M =Co(II), Ni(II), Cu(II) and Zn(II) with x=0 for Cu(II), and 2 for other metal ions, and n =3, 4 or 5. The TG and DTA studies of these compounds show that one or more intermediate compounds are formed in each case before the metal oxides are produced.