The measurement of the temperature of objects by remote means is important in science, industry, and other areas. A technique often employed is ratio radiometry, in which the temperature of the object is related to the ratio of the radiometric signals derived from each of two measurement spectral bands. A lingering question as to whether this technique is valid for determining temperature is answered. Although the technique is generally valid, certain spectral-band pairs can produce ambiguous results, namely, the same ratio can be obtained by looking at two objects, each at a particular, yet different temperature. Ambiguities occur only for spectral-band arrangements in which one spectral band is completely contained within the other. Examples are presented to illustrate this finding.