To find out the mechanism of high thermionic efficiency of one of the widely used electron emitters - LaB6, a comparative study of the state of Lanthanum and Boron atoms in a model double La–B submonolayer film, on one hand, and on separate La and B films - on the other, has been done by detecting the stretch mode of adsorbed CO molecules. The separate La and B films, as well as double La–B films, have been deposited on Mo(110) substrates by thermal (La) and electron bombardment (B) evaporation in an ultra-high vacuum chamber and their growth mode and surface concentration were determined by AES and work-function measurements. An equilibrium La–B film features a work function value very close to the mean value of that for bulk Lanthanum hexaboride crystal - c.a. 2.6 eV, which is much lower than the corresponding values of separate La (3.3 eV) and B (5.8 eV) films. By comparing the high-sensitive stretching intra-molecular modes of probe CO molecules by in-situ reflection-absorption infrared spectroscopy, it was found that the mechanism of this synergistic effect is electrostatic dipolar La–B surface layer formation, rather than the chemical bonding between La and B atoms in a double film. The former is considered as a key factor determining the high thermionic efficiency of bulk Lanthanum hexaboride material.