Hydrodynamical stability of steady flows of two-phase media with a domination of cloud evaporation is studied. Steady state flows dominated by evaporation effects are demonstrated to be intrinsically nonuni-form with characteristic spatial scale inversely proportional to the evaporation mass rate gain. In the short-wavelength limit small perturbations are shown to be stable in subsonic regions of such flows and unstable in supersonic ones. The bulk hydrodynamical motion in broad-absorption line regions of QSOs and in the interstellar medium of the Galaxy are estimated to be influenced considerably by evaporating clouds. As a consequence, stability of these motions is argued to be affected by evaporation effects.