A radon prone geology is one for which the probability is increased that in a house built on it, elevated indoor Rn concentration will be encountered, or that its Rn potential will be increased. Labelling geological units as Rn prone or not can be an important support in deciding whether a geographical or administrative region in which that geological unit occurs, should be called Rn prone area, possibly in absence of other predictors. In this article a method is proposed which, given a set of geological classes, sorts the classes into Rn prone and non-Rn prone classes depending on a classification criterion which one can choose according the purpose. The method is computationally simple and is demonstrated on the example of Germany.