The invention relates to a composition comprising acid magnetic particles (p) based on an iron compound, the acid magnetic particles (p) being complexed by one or more gem-bisphosphonate compounds, of formula I: X-L-CH(PO3H2)2  (I) in which: L represents an organic group connecting the X group to the gem-bisphosphonate group —CH(PO3H2)2; X represents a chemical group capable of reacting with a biovector; all or some of the X groups of the particles optionally being coupled to a biovector. The invention relates, also to a process for the preparation of the compositions and their use, in particular as contrast products for Magnetic Resonance Imaging (MRI).