A caesium filled thermionic diode, working in the space charge limited mode, is used for obtaining further data on the mechanism of photoionization in caesium vapours. Particular emphasis is given to molecule photoionization, in the wavelength range of higher members of the principal series, the corresponding cross-section being estimated at 7· 10−18 cm2 for 3257 Å. The ions formed at a relative large distance (a few cm). from the thermionic detector are found to contribute substantially in the space charge neutralization. Data on the optimization of the cadmium absorption resonance line 3261 Å are also given.