In this work, a detailed single particle model was developed to describe the reaction of porous solids consisting of different solid species, with an initial composition that can be non-uniformly distributed along its radius, and multiple reactions. The continuous particle model is used to describe the rate of reduction of iron-titanium oxide particles for Chemical Looping Combustion processes, considering a two-step reaction mechanism involving two solid reagents: hematite and pseudobrookite. The experimentally observed non-uniform initial distribution of solid species, as a consequence of the solids activation procedure, was accounted for by assuming an initial core-shell structure with a diffused interface with different compositions of the two areas of the pellets. A detailed sensitivity analysis was carried out assuming different kinetics and initial concentration profiles of the different solid species involved. The results confirm that the initial distribution of hematite and pseudobrookite has a major influence on the predicted particle conversion rate. A comparison was carried out with experimental data obtained by thermogravimetric analysis for the reduction of small spherical ilmenite particles (200 mu m) with CO and H-2 at 600-800 degrees C and at atmospheric pressure. The study proves that the model can explain features of the experimental results that do not fit any shrinking core model. (C) 2015 Elsevier Ltd. All rights reserved.