There are a number of systems in which the substrate shows a spectral strong absorption in the region of the characteristic vibration of the surface species. These systems are extremely difficult to study because of the overlapping of a small spectroscopic signal from surface species with a very strong absorption signal from the substrate. For example, major mineral components of hard tissue, carbonates and phosphates, show very strong absorbance in a relatively wide spectral region in which the characteristic vibrations of the carboxylate, amide, C-O, or other groups of soft tissue, having a biological importance, take place. The system chosen for the study was calcite with very strong absorption between 1600 and 1400 cm(-1) due to carbonate vibrations and oleate which forms a calcium oleate surface complex with characteristic carboxylate absorbance bands at about 1500 cm(-1). It is shown, for the first time, that the experimental spectra of the oleate adsorbed species can be recorded successfully, at different incidence angles and two polarizations, in the spectral region of very strong substrate absorption. Moreover, the experimental spectra can be reproduced by spectral simulations, giving detailed information about the surface composition and structure of the produced adsorbed layers even for those with lateral heterogeneous structure. The adsorbed oleate forms surface calcium carboxylate species with two preferential conformations: unidentate-like (a band at 1537 cm(-1)) and bidentate-like (a band at 1575 cm(-1)). At multilayer coverage, the adsorbed layer forms a patchlike structure produced by a pillar growth mechanism. This spectroscopic structural finding was supported by atomic force microscope measurements. The various experimental and theoretical aspects of the infrared external reflection technique within a very strong absorbing region of substrate are also discussed. For comparison, a discussion of the aliphatic stretching vibration region in the recorded spectra, at about 3000 cm(-1), in which calcite is transparent is also performed.