Abstract
The simple order-disorder Ising model for one sublattice is extended to include multiple separate order-disorder processes and their mutual interaction and coupling. The multiple sublattice model shows that the Curie temperature (T
c
) is a weighted average of what would be determined by any of the disorder processes alone. The weighting involves the dipole moments of the individual disordering processes. This additive behavior is explored in detail for two order-disorder ferroelectrics where plentiful solid solution data are available: lithium niobate (LiNbO
3
) and lithium tantalate (LiTaO
3
).