Technical Abstract: Molybdenum adsorption on amorphous aluminum and iron oxides was investigated as a function of solution pH and solution ionic strength. In this study in situ Raman and Fourier transform infrared (FTIR) spectroscopic methods were combined with sorption techniques, electrophoretic mobility measurements, and surface complexation modeling to study the interaction of Mo with amorphous oxide surfaces. The speciation of Mo in aqueous solution was examined using Raman and attenuated total reflectance ATR-FTIR methods as a function of solution pH. Good agreement was found between the vibrational spectra of Mo in aqueous solution and those of Mo sorbed to amorphous Al oxide. The mechanisms of Mo sorption to these surfaces based on the spectroscopic, sorption, and electrophoretic mobility measurements are as follows: Mo forms predominantly inner-sphere surface complexes at low pH and predominantly outer-sphere surface complexes at high pH. These surface configurations were used to constrain the input parameters of the triple layer surface complexation model to describe Mo adsorption on soils. After applying the triple layer model to Mo adsorption on 36 soils, a general regression model was developed for predicting Mo surface complexation constants from five independently measured soil chemical characteristics: cation exchange capacity, organic carbon content, inorganic carbon content, aluminum oxide content, and iron oxide content. The triple layer model was well able to predict Mo adsorption on all the soils at all pH values. The surface speciation predicted by the model for soil was in agreement with that predicted from spectroscopy for Mo adsorption on amorphous Al oxide.