We present a new method for obtaining spectroscopic information about short‐lived autoionizing states: branching ratio spectroscopy (BRS). Under the conditions that the transition moment is dominated by one doubly‐excited state, we show that the BRS signal is independent of the position and width of the excited resonance, but has structure located at the position of any perturbing resonance. Moreover, that structure has the simple form of a Beutler‐Fano lineshape characterized by the position and width of the perturber only. BRS is insensitive to depletion broadening, producing a lineshape which is completely independent of the excitation laser power. This makes it particularly useful for measuring structure in the far wings of a spectrum.