The model of the instrumental function of a fourcircle X-ray diffractometer suggested earlier [Chulichkov et al. (1987), Kristallografiya, 32, 11071114] is complemented by the introduction of the crystal mosaicity. This improved model is used to develop a method for the reconstruction of the mosaicity function profile f(w) from the experimental intensity profile l(w) measured on a diffractometer. The method consists of the reduction of the experimental l(w) distribution to the form it would have if it were measured on a diffractometer with an instrumental function close to the 6 function. The suggested method for determining f(w) is tested on Si crystals with dislocation densities Nd=3×10 ~° and 2x 1011 m -2.