The use of a differential mobility analyzer to perform aerosol size spectrum measurements requires an inversion method to go from the measured sensor responses to the desired size spectrum information. Here we present a linear inversion method that can be run on a microcomputer or a small minicomputer. It does not use some of the approximations made in the techniques currently available and hence gives better inversion accuracy. The method shows good immunity to both random and systematic experimental error. It is applied to numerous test case aerosols.