A small quantum dot containing approximately 20 electrons is realized in a two-dimensional electron system of an AlxGa1-xAs/GaAs heterostructure. Conventional transport and microwave spectroscopy reveal the dot’s electronic structure. By applying a coherently coupled two-source technique, we are able to determine the complex microwave-induced tunnel current. The amplitude of this photoconductance resolves photon-assisted tunneling (PAT) in the nonlinear regime through the ground state and an excited state as well. The out-of-phase component (susceptance) allows us to study charge relaxation within the quantum dot on a time scale comparable to the microwave beat period.