Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4 <-- 0 or 5 <-- 0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a DELTA-upsilon-OH = 1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molescules. Although the 4-nu-(OH) vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge-Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.