It is demonstrated that the temperature of a thermal radiator can be determined without prior knowledge of the emissivity of the source by curve-fit-ting techniques using multiple spectral radiance measurements. This new passive measurement technique assumes only that a smooth function exists between spectral emissivity and wavelength. The spectral radiance values are fitted to a Planck radiation law relation to yield the temperature of the source. Error analysis shows that relative errors in the temperature measurements are generally an order of magnitude less than in the spectral radiance measure-ments and in the simultaneously calculated spectral emissivity values. Computer simulations are included that show the effects of varying different parameters, such as the number of data pairs, the wavelength range, the spectral emissivity behavior, the source temperature, and the measurement noise, on the accuracy of the temperature determination. Experimental confirmation of this technique is presented, showing temperature measurements within 1% of the actual temperatures on a platinum surface within a temperature range of 1273 to 1724 K.