High-frequency electron paramagnetic resonance (HF-EPR) provides a contactless microscopic probe of magnetic impurities and their surroundings in nanoparticles. In 5.0 nm colloidally prepared CdSe quantum dots (QDs) containing 0.6% Mn2+, a core and a surface site can be readily distinguished on the basis of g-factor and Mn-55 hyperfine interaction. In contrast to EPR in the bulk, at low temperatures a broad background signal develops, which at high fields is the only remaining feature. Previous studies have suggested this background signal originates from exchange coupled Mn2+ clusters arising from spinodal decomposition; HF-EPR shows that it is due to strain-broadening of the zero-field splitting related to local lattice distortions within the QD. (C) 2011 Elsevier B. V. All rights reserved.