Co40Pt60 nanoparticles with an average particle size of 3.9 nm were prepared by polyol pathway without incorporating any capping agents or surfactants. The as-prepared nanoparticles were superparamagnetic having disordered A1-face centered cubic structure, which transforms to face centered tetragonal (L1(0)) phase on heat treatment, without the requirement of a reducing gas. The Curie temperature of the Co40Pt60 nanoparticles was determined to be 482 degrees C from the thermomagnetic analysis. Coercivity upto 7 kOe was obtained on annealing at a relatively lower temperature of 600 degrees C. Anomalous variation in coercivity has been observed with annealing. Annealing temperature dependent two phase behavior was studied by delta M measurements. The CoPt samples exhibited dipolar coupling on annealing up to 700 degrees C and strong exchange coupling was evident with further increase in annealing temperature. (C) 2015 Elsevier B.V. All rights reserved.