This paper describes a method for improving the performance of a continuous speech recognizer by modelling pronunciation variation. Although the improvements obtained with this method are small, they are in line with those reported by other authors. A series of experiments was carried out to model pronunciation variation. In the first set of experiments word internal pronunciation variation was modelled by applying a set of four phonological rules to the words in the lexicon. In the second set of experiments, variation across word boundaries was also modelled. The results obtained with both methods are presented in detail. Furthermore, statistics are given on the application of the four phonological rules on the training database. We will explain why the improvements obtained with this method are small and how we intend to increase the improvements in our future research.