Infrared reflection absorption (IRA) spectroscopy was applied to study the adsorption process and structural change of(methacryloyloxy)alkyl dihydrogen phosphates, M10P and M2P (see Figure 1), during their self-assembling process at a silver substrate from a methylmethacrylate solution (5 x 10(-5) mol/L). The saturation coverage of M10P was completed after about 50 min, while that of M2P was completed after about 25 min. The self-assembled monolayers of both M10P and M2P gave the v(s)(PO32-) band around 980 cm(-1), indicating that the dissociation of the dihydrogen phosphate groups takes place on adsorption to the silver substrate. At an initial stage of the self-assembling process of M10P the IRA spectra gave only the v(s)(PO32-) band. After several minutes of incubation there arose the v(as)(PO32-) band near 1080 cm(-1) and the intensity ratio of v(as)(PO32-)/v(s)(PO32-) increased with time, indicating that the axis of the PO32- group of the adsorbate changes from a perpendicular orientation to a tilted one. On the other hand, the IRA spectra of the self-assembled monolayer of M2P gave both the v(s)(PO32-) and v(as)(PO32-) bands, with the onset of adsorption and the intensity ratio of v(as)(PO32-)/v(s)(PO32-) virtually constant. Thus, in contrast to the case of M10P, the PO32- groups of M2P take an almost random orientation during the self-assembling process. The explicit differences in the kinetics of adsorption and structural change between M10P and M2P were explained in terms of a larger mobility of M2P at the substrate surface and/or an interaction between the alkyl moieties of M10P.