To clarify the thermochemical and thermionic properties of various layers of alkali halide (MX) such as LiF, LiI, NaBr, KCl, RbCl or CsCl, a thin film [<()over bar>(0) similar or equal to 1 - 10(4) molecular layers (ML) at the start] of MX was prepared on a metal plate (similar to 0.04 cm(2)), and the absolute desorption rates (n(+) and n(0)) of both positive ion (M+) and neutral molecule (MX0) were measured simultaneously in a vacuum as a function of sample temperature increased stepwise (by similar to5 K, approx. every 20 min) or constantly (similar to0.3-80 K/s) by using our dual-ion source system. Theoretical analysis of the experimental data yields the following conclusions. (1) Below the melting point (T-m), more than 90% of MX are desorbed as MX0. (2) Its desorption energy (E-0) increases up to the heat of sublimation as <()over bar>(0) increases, suggesting that the film prepared at <()over bar>(0) much less than 10(3) ML is partly amorphous rather than crystalline. (3) Tonic desorption, on the other hand, occurs mainly above T-m giving 1-3 peaks in a temperature-programmed desorption spectrum. (4) The desorption energy of M+ from MX is much larger than E-0 and depends upon the species of X. (5) The film surface is very inhomogeneous in thermionic properties, and M+ is desorbed from those active sites having a high work function (e.g. 724 kJ/mol for LiCl). (6) The fractional surface area of the active sites over the him surface is as small as 10(-2)-10(-5). (7) The ionization efficiency (similar ton(+)/n(0)) is less than 10(-2), usually increasing with a decrease in <()over bar>(0).