The bioavailability of poorly-water-soluble active pharmaceutical ingredients (APIs) can be significantly improved by so-called amorphous solid dispersions (ASDs). However, the long-term stability of ASDs might be impaired by API recrystallization and/or amorphous phase separation (APS). So far, no methods have been reported to quantify APS in ASDs. In this work, phase-separation kinetics as well as the compositions of the two amorphous phases evolving due to APS were quantitatively determined for the first time using confocal Raman spectroscopy. Raman spectra were evaluated via non-linear multivariate Indirect Hard Modeling and verified by differential scanning calorimetry and hot-stage microscopy. APS in water-free ASDs of ibuprofen and poly (dl-lactic-co-glycolic acid) was investigated considering the influence of temperature and polymer architecture (linear vs. star-shaped). Water absorbed at 40 °C and 75% relative humidity (RH) promotes APS which was quantified for formulations of felodipine/poly(vinyl pyrrolidone) and ibuprofen/poly(vinyl pyrrolidone).