Stoichiometry and the mechanism of the thermal decomposition of fibrillar isopolytrimolybdates Co(II), Ni(II), Zn(II) and Cu(II) of the following general formulae : CoMo3O10. 5H(2)O, NiMo3O10. 5H(2)O, ZnMo3O10. 5H(2)O, ZnMo3O10. 3.75H(2)O and CuMo3O10. 5H(2)O have been investigated. Water was found to be essential to the structure of these compounds, and with its loss on heating, the compounds became amorphous and (above 300 degrees C) decomposed, forming MMoO4 and MoO3 where M=Co(II), Ni(II), Zn(II) and Cu(II). The change in enthalpy of each process (Delta(deh)H and Delta(dec)H) has been determined.