The conventional pyrometric measurement of wall, electrode, and probe temperatures through an alkali‐vapor atmosphere (which is, in part, cooler than the surface) has been found to lead to significant under‐estimation of the temperature. The error arises from optical absorption of the alkali‐vapor dimers in the region of the pyrometer passband. As an example, the observation through 6 cm of saturated potassium vapor of a surface whose brightness temperature is 2500°K results in an error of from 50° to 500°K as the vapor temperature varies from 300° to 370°C. Thus, substantial errors may be introduced into the determination of properties such as electrical conductivities and reaction rates, which are exponentially dependent upon temperature. The reciprocal‐temperature error was found to be roughly proportional to the dimer optical depth, and agreed with theoretical predictions utilizing dimer optical absorption coefficients measured during these experiments.