The effect of a temperature-dependent solute diffusion coefficient on a model of unidirectional solidification of a binary melt with a quasi-equilibrium mushy (two-phase) zone is studied. The Soret effect (thermodiffusion) is also included in the analysis. The concentration field in the liquid, solid and mushy phases, as well as the rate of solidification and mushy zone thickness, are found analytically as functions of all thermophysical parameters. The role of the nonlinear solute transport is detailed in the analysis. On the basis of analytical solutions, the regime of solidification with a quasi-equilibrium mushy zone is replaced by an equivalent discontinuity surface (frontal) regime with new boundary conditions.