Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) limb observations at 166 and 156 nm have been analyzed, and emission mechanisms at these wavelengths have been modeled. The observations prove to be diagnostic of both the thermal carbon density and the O2 to CO2 mixing ratio. The photons seen by the PVOUVS at 156 and 166 nm arise from a variety of sources: photoelectron impact dissociative excitation of CO and CO2, photodissociative excitation of CO and CO2, photoelectron impact excitation of C, solar resonance scattering by C, and the underlying CO fourth-positive bands (CO4PG). Solar resonant scattering by atomic carbon is an important contribution to the observed intensity and, furthermore, a diagnostic of the O2 to CO2 mixing ratio. An O2 to CO2 mixing ratio of 0.3% yielded the best agreement between the modeled and observed limb intensities for both the 156- and 166-nm data sets. For that O2 to CO2 mixing ratio, the atomic carbon density reaches a maximum at 155 km of 5¿106 cm-3.