The in situ measurements from the Pioneer Venus Orbiter (PVO) Ion Mass Spectrometer have led to the detection of H2 + ions in the Venus ionosphere. Although H2 + is a minor ion, its measurement provides the first direct clue for determining the H2 abundance in the upper atmosphere of Venus. A photochemical model using PVO measurements is used to derive an H2 mixing ratio of 10 ppm below 140 km altitude. The presence of this much H2 confirms our previous prediction that the reactions of O+ with H2 and subsequent recombination of OH+ provide an important source of nonthermal H observed in the Venus exosphere. The estimated escape flux for H is ~108 cm-2 s-1.