  We describe theoretically the depairing effect of a microwave field on
diffusive s-wave superconductors. The ground state of the superconductor is
altered qualitatively in analogy to the depairing due to a dc current. In
contrast to dc-depairing the density of states acquires, for microwaves with
frequency $\omega_0$, steps at multiples of the photon energy $\Delta\pm
n\hbar\omega_0$ and shows an exponential-like tail in the subgap regime. We
show that this ac-depairing explains the measured frequency shift of a
superconducting resonator with microwave power at low temperatures.
