Using the FORC method, we present in this communication a study on the diluted spin transition materials with the formula [FexM1 - x(btr)2 (NCS)2] · H2O with M = Zn, Co and Ni. We compare the evolution of distributions obtained for different dilutions with the three impurities and we notice that these distributions depend not only on the dilution (1 - x) but also on the impurity used. The statistical analysis shows that the maximum values of the distributions are close to a linear dependence for Zn and Co. The impurity that does not obey this dependence is Ni, which volume is close to the average volumes of high spin and low spin iron atoms. In addition, opposite to others, the complex with Ni impurities presents a rotation of the principal axes. © 2006 Elsevier Ltd. All rights reserved.