On nitriding ferritic Fe-M (M = Ti, V, Cr or Mo) alloys, three types of absorbed nitrogen atoms can be distinguished: (i) nitrogen incorporated in the stoichiometric nitride; (ii) nitrogen adsorbed at the nitride/matrix interface, and (iii) nitrogen dissolved in octahedral interstices of the ferrite lattice. As compared with pure ferrite, enhanced nitrogen lattice solubility is observed for the ferritic matrix of Fe-M alloys. This excess nitrogen can be ascribed to an average lattice dilation induced by the volume misfit between nitrides and matrix. A model is developed for a quantitative description of the amount of excess nitrogen in ferrite. The treatment originates from an application of a theory due to Eshelby, developed originally for the case of elastic distortions by point imperfections, in conjunction with the thermodynamics of stressed solids. The model is applied to published data on excess nitrogen in various Fe-M alloys where coherent nitrides develop. The dependence of the amount of excess nitrogen in Fe-M alloys on the type of alloying element can be explained in terms of an interaction parameter, weighing up the contributions of chemical and strain energy changes occurring on precipitation of nitrides.