The stability of the supersaturated state in PbO-B2O3-Fe2O3 fluxed melts used for the LPE growth of thin magnetic garnet films is investigated. The data show that these melts are very stable against spontaneous nucleation of the garnet phase, especially when undercooling is only moderate. Constant supersaturation can be maintained for 200 min, even with 100°C undercooling. Once spontaneous nucleation of garnet has occurred, the decay of the supersaturation, i.e., rate of ageing, is well-defined and reproducible. The rate of ageing increases with increased undercooling. © 1975.