A model of the head echo is examined, in which, as a source of ionization and recombination, an interaction layer of impinging atmospheric molecules (O2) mixing with reflected ones is assumed. Dissociative recombination is considered as the most probable process. If we suppose that the interaction layer is that arising in nearly free molecular flow with no shock wave present, we obtain, using the observed heights of head echoes, dimensions of meteoroids in the range 10–1 cm for absolute magnitudes between −10 and −2. It is shown that such a layer may be used to explain the high initial recombination coefficient of the order of 10−5 cm3/sec. Using the ionization collision cross-sections measured by Utterback (1963) we obtain for the mean extent of the interaction layer a value 1–100 m for heights 80–130 km, in accordance with radar observations.