<p>This study examines the rejuvenation mechanisms in bituminous RAP mastics using Fourier transform infrared (FTIR) spectroscopy and multivariate discriminant analysis. The chemical interactions between RAP binders, virgin bitumen, warm mix additives, and two commercial rejuvenators (RJs) were evaluated to determine their effectiveness in restoring aged binders on the chemical level. A hybrid approach combining partial least squares regression and linear discriminant analysis (PLSR–LDA) was applied to extract latent variables, classify samples, and identify critical wavenumbers associated with rejuvenation. The findings indicate that Sylvaroad mitigates oxidation effects, particularly around ~ 1770–1725&#xa0;cm⁻¹, while Storflux primarily influences methyl bending near 1370–1360&#xa0;cm⁻¹. Despite these effects, irreversible oxidative aging remains evident in the 1300–1160&#xa0;cm⁻¹ range, linked to oxygen-containing compounds, suggesting that complete restoration of binder chemical alteration is unattainable. Additionally, mineral composition markers, particularly in the 489–446&#xa0;cm⁻¹ and 590–555&#xa0;cm⁻¹ ranges, persist as key indicators for distinguishing varying RAP contents after rejuvenation.</p>