The spectral dependence of the photogeneration efficiency. η in sublimed undoped C60-C70films has been determined from photoinduced xerographic discharge curves. The dependence of η on the exciting light wavelength from 7250 to 3500Å and electric field are reported. At an applied field of 2–7 × 105V cm−1η is essentially wavelength independent with a value of about 5 × 10−3from 7250 to 5500Å. At shorter wavelengths, η is essentially proportional to the absorption coefficient, rising by more than an order of magnitude to about 6 × 10−2at 3500Å. The difference between the charge-transfer efficiencies of these two regions is discussed in terms of an Onsager surface-mediated dissociation of singlet excitons.