An Al vapour cloud (~1 μg) is generated between Al electrodes of a 10 kV transient spark (1 μs duration) in an environmental atmosphere of pure oxygen. During the cooling-down period of the so formed Al/O-plasma, the reaction Al + O + M -> AlO + M is observed. It can be shown that the conditions within the limited reacting gas volume may be considered as quasi-stationary and hence the laws of chemical equilibrium for computing number densities of atoms and molecules may be applied. The emissivity εrel (radiation per unit volume) of the AlO B2 Σ → 2Σ system at 4842 Å and the gas temperature of the decaying plasma are recorded in terms of a laboratory time scale. The maximum of εrel (normalized maximum) may be correlated to the dissociation energy of AlO, yielding 5.17 ±0.3 eV. Effects of dilution, the pressure influence as well as the vibrational relaxation of AlO during its formation are considered and included in the calculations.