Worldwide high-grade nickel sulphide deposits are being depleted and new ones are not being discovered. Therefore, the exploitation of low-grade ultramafic nickel ores is of increasing interest. Processing these deposits, however, is a challenge, due to the high serpentine content. Serpentine minerals are anisotropic, increase the viscosity of ore slurries, dilute the concentrate, and slim-coat the valuable nickel mineral pentlandite. Microwave pre-treatment of ultramafic nickel ores has been proposed to convert the serpentine to olivine, reduce the viscosity of the ore slurry, improve ore grindability and mineral liberation, and reduce the overall energy requirements of processing. To optimize the process, an understanding of the microwave heating properties of ultramafic nickel ores and constituent minerals is necessary. The microwave heating of serpentine and olivine, the primary components of ultramafic nickel ores, is explored in this paper. Microwave heating tests and high-temperature microwave properties (real and imaginary permittivity) analysis were performed. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) tests were also completed to explain the permittivity data. Serpentine responded more readily to microwaves than olivine and the maximum temperatures achieved were 216 degrees C and 57 degrees C, respectively. Serpentine did not reach the temperature required for dehydroxylation (600-700 degrees C). The real and imaginary permittivities for serpentine and olivine varied with temperature, but overall were low, indicating these minerals cannot be expected to heat well on their own upon exposure to microwave radiation. A microwave susceptor is required for the serpentine in ultramafic nickel ores to reach dehydroxylation temperatures. It is hypothesized that the small quantities of pyrrhotite and magnetite present in ultramafic ores play this role. (C) 2018 Elsevier B.V. All rights reserved.