To elucidate the mechanism of thermal desorptions of Li+ and LiBr from a thin film (0.2-2250 molecular layers) of lithium bromide deposited on a platinum surface, both ionic and neutral desorption rates were simultaneously measured as a function of heating rate (0.4-24 K/s). Theoretical analysis of the spectra thus obtained with the desorptions from heterogeneous surfaces with respect to work function shows that (1) desorption of LiBr exhibits three peaks corresponding to activation energies (E) of 139, 344 and 377 kJ/mol owing to desorptions from non-active sites (low work function spots) on the surface of LiBr or Pt, (2) desorption of Li+ shows two peaks (E = 375 and 535 kJ/mol) due to desorption from active sites (high work function spots) on the inhomogeneous surface of LiBr itself or Pt, and (3) the fractional surface area of the active sites over each surface is as small as similar to 10(-2)-10(-3).