The nature of adsorbed calcium carboxylates (oleate and stearate) has been investigated by in situlex situ Fourier transform infrared internal reflection spectroscopy (FTIR/IRS). Experimental results confirm that chemisorbed carboxylate is characterized by a singlet band near 1550 cm(-1) while physisorbed calcium dicarboxylate salt is characterized by a doublet band near 1540 and 1575 cm(-1). The singlet IR band at 1550 cm(-1) was attributed to the carboxylate groups associated with calcium ions in a bridging mode. On the other hand, the doublet IR absorbance band at 1540 and 1575 cm(-1), which is observed for adsorbed carboxylate groups at multilayer coverage and for bulk-precipitated calcium dicarboxylate salts, was assigned for the carboxylate groups associated with calcium ions in unidentate and bidentate modes, respectively. In this regard, the IR singlet at 1550 cm(-1) and the doublet at 1540 and 1575 cm(-1) are correlated to the coordination structure of carboxylate groups with calcium ions. Constrained carboxylate groups at a surface such as a chemisorbed layer at a calcium semisoluble mineral surface (carboxylate bridging coordination at the fluorite surface, for example) and/or a transferred Langmuir-Blodgett (LB) layer at a high-transfer pressure give rise to the IR singlet at 1550 cm(-1). Carboxylate groups associated with calcium ions in three-dimensional seven- or eight-fold coordination, which may contain carboxylate groups coordinated with calcium ions in different modes (i.e., unidentate and bidentate modes) are responsible for the IR doublet at 1540 and 1575 cm(-1). This assignment is in agreement with the observed IR doublet for bulk-precipitated calcium dicarboxylate salts, adsorbed/surface precipitated calcium carboxylate species at multilayer coverage, and transferred calcium distearate at a low-transfer pressure because under these conditions formation of the three-dimensional structure is possible.