Amorphous iron nanoparticles obtained by the sonication of Fe(CO)(5) in decalin were coated by various surfactants using the self-assembled monolayer (SAM) technique. They have been shown previously to behave as superparamagnetic materials. These measurements are now extended to include information regarding their blocking temperature (T-B). The results show that for one functional group T-B is only slightly dependent on the length of the alkyl chain. On the other hand, the nature of the functional group bonded to the iron particles is strongly affecting T-B. (C) 2002 Elsevier Science B.V. All rights reserved.