Abstract: Transition from microwave enhancement of supercurrent to superconductivity suppression is investigated in impure superconductors. It is demonstrated that the frequency range of the enhancement effect narrows with the decrease of the electron mean free path, l, and at l ⩽ 1 nm electron heating is observed in the whole frequency range. Dependences of frequency boundaries on l are explained by taking into account strong electron-electron interaction in impure metals.