With the aim of characterizing ground preparations of paintings by infrared reflection spectroscopy, the CaSO₄–H₂O system (gypsum/bassanite/anhydrite) has been re-investigated, evaluating and assigning the SO₄^2- and OH overtone and combination bands, respectively, in the ranges 1900–2700 cm⁻¹ and 5000–6000 cm⁻¹ resulting from reflection and high concentration transmission spectra. The second-order modes have been proven to be highly specific, reliable, and less affected by overlap with bands of organic binders and can hence be exploited for the identification of the sulfate hydration phase using infrared (IR) reflection spectroscopy. Subsequently, the characterization and identification of hydration phases in unknown sulfate-based ground preparations on authentic artworks have been carried out noninvasively by fiber-optic reflection IR spectroscopy and on cross-sections by infrared reflection micro-spectroscopy. The spectroscopic data collected both on standards and artworks have been cross-validated by X-ray diffraction.