Quenched metastable wustites are shown to undergo a two-stage disproportionation reaction on heating. A mixture of nearly stoichiometric iron monoxide and magnetite is formed during the first stage, which takes place at approximate to 470 K. The resulting nearly stoichiometric Fe1-yO remains metastable up to approximate to 530 K. Above this temperature the stable two-phase mixture of iron and magnetite is slowly formed. The thermodynamic and kinetic aspects of this two-stage disproportionation reaction have been studied in a step-wise heated adiabatic calorimeter. The decomposition behaviour is rationalized in a Gibbs energy of formation representation of stable and metastable phases in the iron-oxygen system. The antiferromagnetic to paramagnetic order-disorder transition which takes place in Fe1-yO at approximate to(1)95 K is found to be greatly influenced by the oxygen content; it becomes much more cooperative as the exact 1:1 stoichiometry is approached.