Two-dimensional nonisotropic flows of perfect polytropic gas are considered, which occur when the smooth surface that separates the given flow of gas from vacuum is instantly removed. Solution of the problem of such discontinuity disintegration is constructed in the space of special variables in the form of convergent characteristic series. On the basis of investigation of the series convergence region it is proved that gas particles at the boundary with vacuum continue moving for some time each along its straight path at its constant velocity. Next, the case of continuous contiguity of gas through the smooth free surface to vacuum is considered. It is shown that up to the instant of occurrence of an infinite gradient at the free boundary, or up to the instant of local gas focusing, the derived law of motion of the free boundary remains valid. This may be used as the boundary condition in numerical solution of problems on discharge of perfect gas into vacuum. A system of transport equations is obtained and investigated, which defines the behavior of the gradient of gas dynamic parameters at the boundary with vacuum.