Specific absorption rate (SAR) heating using radiofrequency (RF) waves is affected by the RF frequency and amplitude, and the conductivity of the tissue. Recently, conductive nanoparticles were demonstrated to induce hyperthermia in vitro and in vivo upon irradiation with an external 13.56 MHz RF field. The addition of conductive nanoparticles was assumed to increase the tissue conductivity and SAR. However, no quantitative studies have been performed that characterize the conductivities of biocompatible colloids or tissues containing nanoparticles, and relate the conductivity to SAR. The complex permittivities were measured for colloids containing single-wall carbon nanotubes (SWCNTs) in normal saline with 0.32% w/v Pluronic F108 nonionic surfactant. The carbon concentrations of the colloids ranged from 0 to 88 mM. The permittivities were measured using a dielectric probe and RF network analyzer for RF frequencies from 200 MHz to 3 GHz. The nonionic surfactant was added to the colloids to minimize flocculation of the nanotubes during the RF heating experiments. The results were compared with prior measurements of colloids containing 0.02% Pluronic F108. The dielectric and conductivity of the 0.02% Pluronic colloids rose linearly with carbon concentration but the 0.32% Pluronic colloids varied from linearity. Based on the permittivity results, selected colloid samples were placed inside a Bruker 7T/20 magnetic resonance (MR) imaging (MRI) system and irradiated at 300 MHz using a high duty cycle RF pulse sequence. The temperature changes were measured directly using fiber-optic thermometers and indirectly using MR thermometry and spectroscopy. Temperature changes were consistent with the colloid conductivities.