We are concerned with supersonic vortex sheets for the Euler equations of compressible inviscid fluids in two space dimensions. For the problem with constant coefficients we derive an evolution equation for the discontinuity front of the vortex sheet. This is a pseudo-differential equation of order two. In agreement with the classical stability analysis, if the Mach number satisfies < √ 2, the symbol is elliptic and the problem is ill-posed. On the contrary, if > √ 2 then the problem is weakly stable, and we are able to derive a wave-type a priori energy estimate for the solution, with no loss of regularity with respect to the data. Then we prove the well-posedness of the problem, by showing the existence of the solution in weighted Sobolev spaces. K E Y W O R D S compressible Euler equations, contact discontinuities, linear stability, loss of derivatives, vortex sheet, weak stability M S C ( 2 0 1 0 ) 35L50, 35Q35, 76E17, 76N10 √ 2 and are violently unstable when < √ 2, while planar vortex sheets are always violently unstable in three space dimensions. This kind of instabilities is the analogue of the Kelvin-Helmholtz instability for incompressible fluids.Artola and Majda [1] studied certain instabilities of two-dimensional supersonic vortex sheets by analyzing the interaction with highly oscillatory waves through geometric optics. A rigorous mathematical theory on nonlinear stability and local-in-time existence of two-dimensional supersonic vortex sheets was first established by 9] based on their linear stability results in [7] and a Nash-Moser iteration scheme.Characteristic discontinuities, especially vortex sheets, arise in a broad range of physical problems in fluid mechanics, oceanography, aerodynamics, plasma physics, astrophysics, and elastodynamics. The linear results in [7] have been generalized to cover the two-dimensional nonisentropic flows [15], the three-dimensional compressible steady flows [25,27], and the two-dimensional two-phase flows [18]. Recently, the methodology in [7] has been developed to deal with several constant Mathematische Nachrichten. 2020;293:945-969.