A system is described and evaluated for the simultaneous measurement of the intrinsic optical properties of tissue: the scattering coefficient, the absorption coefficient, and the anisotropy factor. This system synthesizes the theory of two integrating spheres and an intervening scattering sample with the inverse adding-doubling algorithm, which employs the adding-doubling solution of the radiative transfer equation to determine the optical properties from the measurement of the light flux within each sphere and of the unscattered transmission. The optical properties may be determined simultaneously, which allows for measurements to be made while the sample undergoes heating, chemical change, or some otherexternal stimulus. An experimental validation of the system with tissue phantoms resulted in the determination of the optical properties with a < 5% deviation when the optical density was between 1 and 10 and the albedo was between 0.4 and 0.95.