The present paper reports on a new experimental technique, the cluster isolated chemical reaction technique. A single pair of reactants (or a controlled number of reactants) is isolated on a large atomic or molecular cluster. The reactants are free to migrate and to collide with each other on the cluster surface. The collision between two reactants results in a chemical reaction and yields observable signals, the cluster playing the role of a reaction medium of finite size. The CICR technique is used here to study the reaction Ba2 + N2O on (N2)8000 clusters and the reaction Ba2 + CO2 on (Ar)5500 clusters. Both these reactions are chemiluminescent and yield rather unstructured emission spectra. The power of the CICR method is exemplified here since it allows unambiguous determination of the emitter as Ba2O in both reactions. This allows us to identify, then, the yet unassigned chemiluminescence spectrum observed by West et al. (J. Chem. Phys.1977, 66, 2139) in flames of Ba/CO2 and Ba/CO mixtures.