Iron (Fe) and aluminum (Al) hydroxides (Al(OH)­₃, Fe(OH)₃) and their mixtures are low-crystalline colloids of less than 100 nm in particle size existing abundantly in soils and natural waters. These mixed hydroxide colloids are of great environmental importance due to their ability to retain and transport nutrients and contaminants. The reactivity and, therefore, the efficiency of these hydroxides in the retention of soil chemicals vary upon aging and might be a result of different chemical composition (Fe/Al ratios), which can define the crystallization rate and pathway. Since it is still a challenge to identify and quantify mineral phases in poorly-crystalline colloids, the objective of this work was to synthesize mixed Fe-Al hydroxide nanoparticles with a wide range of Fe/Al ratios and to resolve the structural transformations in these nano-colloids as they age. ATR-FTIR (Attenuated Total Reflectance Fourier Transformed Infrared) spectroscopy reveals distinct short-order mineral signatures within the co-precipitates while synchrotron-based XRD provides complimentary long-range information. Our XRD and ATR-FTIR data are in good agreement showing that the intensity of goethite (FeOOH) bands increases with time for samples containing 0 and 2% Al substitution. Intermediate Al concentrations (10-20%) retard goethite formation and promote the formation of less crystalline, intermediate structures. Gibbsite (Al(OH)₃) microcrystalline structures appear at 25% Al substitution within the first days of aging at 25°C and infrared band intensities progressively increase for suspensions with higher % Al. For the first time, we applied multivariate curve resolution (MCR) statistical analysis of infrared spectra to environmentally important mixed Fe-Al colloids in order to quantify mineral phases in mixtures. Our data and experimental approach can be used for modeling environmental behavior and long-term reactivity of mixed phase colloids in soils and natural waters.