A new principle of studying the mobility spectrum versus agening of atmospheric ions has been developed. By exposing generated ions to an airstream and a counteracting adjustable electric field, the level when the force of the electric field will be greater than the mechanical (represented by the airstream) can be stated. By measuring the ion current versus the electric field at a fixed air stream, the mobility spectrum can be deduced. By selecting different values for air speed, electric field, and traveling distance for the ions, the mobility spectrum versus agening of the ions can be deduced. Examples of recordings together with analysis are given. ¿ American Geophysical Union 1994