Raman spectroscopy was used to investigate Al selectivity in Si-doped tetramethylammonium hydroxide(TMAH) solutions whose Si dopant concentration covers both Al passivating and Al nonpassivating instances. Raman spectra of the Si doped solutions suggest different dominant ionic silicate species for passivating and nonpassivating casts: octamers in the nonpassivating case and cyclic trimers, tetramers, and oligomers in the passivating case. pH measurement results on both Al passivating and Al nonpassivating TMAH solutions suggest that pH cannot be considered as the sole parameter affecting aluminum selectivity. Al dissolution was observed in the solutions doped below the Al passivating concentration, while no Al dissolution occurred in the solutions doped above the Al passivating concentration, consistent with reported results. Al(OH)(4)(-) ions were detected as products of the Al dissolution in the nonpassivating solutions which suggests Al dissolution chemistry in TMAH resembling Al dissolution chemistry in other alkaline solvents. A surface coating layer was observed on the surface of undissolved aluminum. Raman measurements indicate that the coating layer consists primarily of vitreous silica with some embedded nonpolymerized Si-O-Al units. The silica coating lever is suggested as the Al protective layer against TMAH etching.