Stable covalent functionalization of carbon-encapsulated magnetic nanoparticles by the 1,3-dipolar cycloaddition of nitrile oxide is reported. The proposed functionalization route is fast, occurs at mild conditions, and results in high coverage degrees (12-24 wt.%). For comparison, functionalization has also been conducted on C-60 fullerene and multi-wall carbon nanotubes. (C) 2009 Elsevier Ltd. All rights reserved.