© 2014 Elsevier B.V. All rights reserved. The temperature dependence of microwave absorption was investigated for Ni1-yO particles over the frequency range 2.0-13.5 GHz and temperature range 25-1000 °C. Using a coaxial transmission line method with a network analyzer, both the real and imaginary parts of the relative permittivity (ε′r and ε″r, respectively) and permeability (μ′r and μ″r, respectively) were measured; finding that both are largely dependent on the temperature at all frequencies. Furthermore, permeability loss factors related to shape effects were observed at high frequencies, indicating an increase in the microwave-absorption properties. A modified form of Mie's theory was applied to discuss these effects, wherein a spherical model demonstrating a close fit to the shape effect data suggests a more complex microwave-absorption behavior at increased temperature.