Heating of nanoparticles (NPs) using an AC magnetic field depends on several factors, and optimization of these parameters can improve the efficiency of heat generation for effective cancer therapy while administering a low NP treatment dose. This study investigated magnetic field strength and frequency, NP size, NP concentration, and solution viscosity as important parameters that impact the heating efficiency of iron oxide NPs with magnetite (Fe3O4) and maghemite (γ-Fe2O3) crystal structures. Heating efficiencies were determined for each experimental setting, with specific absorption rates (SARs) ranging from 3.7 to 325.9W/g Fe. Magnetic heating was conducted on iron oxide NPs synthesized in our laboratories (with average core sizes of 8, 11, 13, and 18nm), as well as commercially-available iron oxides (with average core sizes of 8, 9, and 16nm). The experimental magnetic coil system made it possible to isolate the effect of magnetic field parameters and independently study the effect on heat generation. The highest SAR values were found for the 18nm synthesized particles and the maghemite nanopowder. Magnetic field strengths were applied in the range of 15.1–47.7kA/m, with field frequencies ranging from 123 to 430kHz. The best heating was observed for the highest field strengths and frequencies tested, with results following trends predicted by the Rosensweig equation. An increase in solution viscosity led to lower heating rates in nanoparticle solutions, which can have significant implications for the application of magnetic fluid hyperthermia in vivo. •Heating was tested in seven iron oxide nanoparticles for different magnetic fields.•Confirms an optimal nanoparticle size for heating that agrees with the literature.•Verifies Rosenweig's equation to predict the effect of field frequency on heating.•Reports reduced heating in high viscosity environments.