Electronic Journal of Differential Equations, Vol. 2003(2003), No. 22, pp. 1-28. Title: Non-classical phase transitions at a sonic point Author: Monique Sable-Tougeron (Univ. de Rennes 1, France) Abstract: The relevant mathematical features of phase transition for a general hyperbolic nonlinear system near a sonic discontinuity are clarified. A well-posed Riemann's problem is obtained, including non-classical undercompressive shocks, defined by a geometrical kinetic relation. A counterpart is the geometrical rejection of some compressive shocks. The result is consistent with the structure profiles of the elasticity model of Shearer-Yang and the combustion model of Majda. Submitted September 5, 2002. Published March 7, 2003. Math Subject Classifications: 35L65, 35L67, 74B20, 76L05, 80A32. Key Words: Hyperbolic; phase transition; Chapman-Jouguet regime; kinetic relation.