Controlled adjustment of the bubble domain collapse field has been demonstrated in epitaxial garnet films of nominal composition Y2.6Sm0.4Ga1.2Fe3.8O12using a thermal annealing technique to redistribute gallium ions between the tetrahedral and octahedral sites in the garnet lattice. This technique involves annealing each film at two different temperatures, the first of which establishes equilibrium at a known temperature. Analysis, based on bubble domain theory, shows that the measured parameters after thermal equilibration can be used to calculate the change in saturation magnetization which will lead to the desired values of the bubble domain collapse field. The change in magnetization can be related to the required change in the equilibrium annealing temperature,Ta, from the derivatived(4&pgr;Ms)/dTa. The value ofd(4&pgr;Ms)/dTahas been found to be approximately constant for900≲Ta≲1200 °Cand is equal to0.35≲G/°C. By using this technique, the bubble domain collapse field can be changed by more than 10 Oe with a precision of a few tenths of an Oersted if the annealing temperatures are controlled to a fraction of 1°C.