Approximate solutions are obtained for the temperature distribution and rate of phase change for the transient one dimensional solidification with mushy zone of a finite slab. The model used here is a generalization of several solidification models with mushy zone and super-saturated liquid phase (l.ph.) from the cases of first kind to the one of third kind conditions at the outer boundary. The proposed solution represents an extension of the Megerlin approximation to solidification with mushy zone. The Megerlin condition for fulfilling the diffusion equations was applied to Goodmann's thermal penetration thickness in order to solve the l.ph. case at super-saturation. (C) 2003 Elsevier Ltd. All rights reserved.