Plasma volume sources have been used very successfully to produce positive or negative ion beams for many applications. The development and testing of a 2.51 plasma source working in pure caesium vapour is described. Measurements of plasma density and electron temperature were made using a planar Langmuir probe and the results are presented. The pure caesium discharge apparently differs from conventional gaseous discharges in the roles of the primary and thermal electrons, due to the large cross sections for collisions with the neutral caesium atom. A model which attempts to describe this behaviour is discussed and compared with the probe data.