An airborne infrared radiation thermometer (IRT) used to measure the sea-surface temperature is described, and the basic radiation principles on which the operation of the instrument depends are discussed. The environmental factors which may tend to introduce errors into the measurement of sea-surface temperature by the IRT are investigated: first by reviewing the works of some other authors; secondly by empirical means using field data supplied by the U.S. Naval Oceanographic Office and the Sandy Hook Marine Laboratory. The empirical results indicate that absorption and emission of infrared energy by atmospheric water vapor are the important physical phenomena which cause IRT error. The results suggest that with increasing values of the combination of sea level mixing ratio and air temperature, IRT error decreases. An attempt is made to explain these results on the basis of radiation principles previously described. (Author).