Abstract Different phenomenological approximations for the analysis of the primary X-ray extinction due to homogeneously and randomly distributed dislocations have been discussed. It has been shown that the primary extinction coefficient calculated according to the models proposed by Ivanov et al. and Olekhnovich is in satisfactory agreement with the experimental data for silicon single crystals with a dislocation density of up to 106 cm−2, in which no correlation between dislocations exists. The model of randomly distributed dislocations proposed by Molodkin was also analysed and improved.