A pinhole camera at the periphery of the cyclotron was used to record neutral products of dissociation of molecular ions accelerated to various radii. From the numbers of particles reaching different parts of a C2 nuclear plate in the camera, the attenuation of the accelerated ions due to collisions with residual air molecules could be deduced. This was compared with the attenuation for H₂+ ions observed in a similar way. The cross section for dissociation of these by air is known, so the cross sections for other ions could be determined. At a velocity corresponding to 1 MeV per nucleon, the cross sections for D^- and HeH+ were found to be respectively 7.6×10^-16 cm² and 1.0×10^-14 cm² with an accuracy to about 30%.