Abstract
An effective-medium approximation is proposed for thermal phase transitions. For quenched bond disorder in the Potts model, the exact critical line is obtained when s=1; for s=2 (Ising), some exact results in the dilute limit and the correct behaviour for Tc(p) near pc are recovered. The critical concentrations are in good agreement with the known values, in both two and three dimensions. For s>4 (first-order transition) the phase diagram shows a two-phase region.

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