The partial pressure of cesium over the cesium antimonides whose compositions were Cs2.95Sb, Cs3.00Sb, and Cs3.25Sb was measured at various temperatures ranging from room temperature to about 180°C. The experimental tube consists of a spherical glass envelope and an ion gauge. The sample was prepared on the inner surface of the spherical envelope. The partial pressure of the cesium dissociated thermally from cesium antimonides was measured by Langmuir's positive ion method. The composition of the samples was determined by comparing their electrical resistivities and thermal activation energies associated with electrical conductivity with those of the cesium antimonides, which had a known composition. The preparation of the sample and the measuring method of the partial pressure of cesium are described precisely. The partial pressure of cesium over cesium antimonides was measured as a function of the absolute temperature, and the results showed that it was represented by the equation log10p(mm Hg) =A — B/T, whereAandBare constants. In the case of Cs3.00Sb, the values ofAandBwere 9.040 and 6300 deg, respectively.