In a cascade impactor each stage collects some particles which are outside its assumed range of sizes and fails to catch some particles which are within its assumed range. This difficulty exists in all non-ideal size-classifying instruments and is designated as a problem in “cross-sensitivity.” A general theory for making a first-order correction was first described by O. Preining and is adapted specifically to cascade impactors in this work. A discussion of the method of making the correction and an application to real and hypothetical aerosols are presented.