A method for measuring the emissivity of a surface using heat flux sensors is described. The emissivity is calculated by directly measuring the heat flux passing through the surface using a heat flux sensor. Unlike calorimetric techniques, it does not require accounting for parasitic heat losses or knowing the temperature history of the sample. This technique allows emissivity measurements of newly developed variable emissivity surfaces, including electrostatic devices which cannot be directly measured using optical techniques. It can measure both passive and active thermal control coatings, and can evaluate many surfaces on the same substrate simultaneously. An experimental setup is detailed and results are presented for emissivity measurements of both active and passive surfaces using commercially available heat flux sensors. Errors are estimated for these measurements. A space-based experiment is also described and results of pre-flight testing are presented.