Different molar ratio of zirconia : mullite precursors have been derived by adjusting the composition of mixed hydroxides hydrogel in ZrO 2-Al 2O 3-SiO 2 system. Kinetics of multistage thermal dehydration of such co-precipitated hydrogels has been studied in relation to composition of as prepared gel under isothermal conditions. The importance of the compositional effect on the kinetics of thermal decomposition of multi-component hydrogels is apparent from the variation of extent of validity of the first order law, values of rate constants and activation energies at the initial and final stages of dehydration reaction.