The microwave absorption characteristics of xylene as a model nonpolar solvent are dramatically increased by the incorporation of dispersed cobalt and magnetite nanoparticles. The addition of 1-2 vol % of these colloids to xylene can produce heating rates by microwaves at 2.45 GHz that approach those for water. The particles have diameters of 5-20 nm and contain a coating on their surface to avoid their aggregation and precipitation from solution. The small particle sizes are compatible with a rapid process of heat transfer to the surrounding xylene, thereby minimizing the generation of large temperature gradients around the particles. Cobalt particles are more effective than magnetite particles for enhancing the heating rates of xylene by microwaves, with nanoparticles of cobalt with diameters less than 10 nm exhibiting greater levels of microwave absorption enhancement than nanoparticles of larger diameters.