An attempt has been made to measure the absorption cross sections of O2 in the metastable state a1Δg from 1087 to 1700 Å. The absorption is measurable (σa ≥ 3 × 10−19 cm2) from around 1500 Å towards shorter wavelengths. Many band structures overlaying various continua have been seen. Absorption cross sections of O2 in the ground state X3Σg− also have been measured in the same wavelength region and our results agree with previously published values within experimental error. The cross sections of O2(a1Δg) are generally larger than those of O2(X3Σg−) below 1290 Å except at the wavelengths of a few strong bands of O2(X3Σg−).