Diffusion equations for two components are set up with mutual drag terms included. It is found that these terms become important when the flux of an escaping gas is of the order of its diffusion limit. In this situation, other gases can be dragged along (blowoff) and both scale heights tend towards equality. It appears that this may occur on Io, giving a strong tendency to equalize the scale heights of O, S, and SO2 without any appeal to eddy diffusion. Ionic bombardment of the exosphere may therefore eject material with an O:S ratio approaching 2, rather than the much higher value expected for a diffusively separated atmosphere. Future aeronomical studies of this atmosphere should include the drag term in the diffusion equations.