The aim of this chapter is to present the necessity of using non-equilibrium thermodynamics to model phase transitions. The condition at the liquid–solid interface during crystallisation is presented. As an example we review the cases of MgSO4·7H2O crystal growth from aqueous solution and ice growth from pure water on cooled surfaces. Temperature jumps at the MgSO4·7H2O–MgSO4 solution and ice–water interfaces were detected, from 0.2 °C and up to 1.7 °C respectively. These observations are experimental proofs for the existence of an interfacial temperature jump during liquid–solid phase transitions. The excess entropy production for heat and mass transport into, out of, and across the interface were used to define the fluxes and forces of the system. Coupled heat and mass flux equations from non-equilibrium thermodynamics were defined for crystal growth and all interface transfer resistivities were determined for both cases. The coupling coefficient showed that, during MgSO4·7H2O and ice crystal growth, around (20 to 30) % of the enthalpies of crystallisation were returned into the liquid. This knowledge can improve film or fugacity models for the interface, change current modelling of phase transitions and eventually help the prevention of crystal growth at unwanted locations in process industry saving considerable capital and operational costs.