[en] The ratio of comparison values of initial and final products is estimated for consideration of phosphiding processes (phosphiding agents being phosphorus or phosphines) for rare earths and their oxides from the same point of view and for conclusion on the properties of films appearing on the surface of so lid particles. The volume taken by one atom (in this case lanthanide atom) with its closet environment, which can vary in the process of reaction is taken for a volume. It is established that in the case of phosphiding of rare earth metals on the surface of particles dense films of monophosphides and polyphosphides appear that prevent the process of uniform distribution of phosphorus in grain and formation of stoichiometric monophosphide. The exception is presented by europium and yttrium for which monophosphide film does not cover the whole surface of metal. In the case of phosphiding of sesquixides the solid monophosphide film is softened by the reaction product, which provides the preparation of the product less variable in composition