In situ analysis of phase separa on behavior during drug release is challenging but can be highly informa ve. Yang and coworkers probed the role of phase separa on and morphology on release behavior of ritonavir-PVPVA ASD tablets in buffer with different drug loadings at 10% intervals. They employed hydrophobic and hydrophilic fluorescent marker molecules and in situ confocal fluorescence imaging to indirectly track gel layer development from the matrix-buffer interface in real me, together with associated drug-polymer phase separa on. They observed rapid phase separa on into separate drug and polymer rich domains. At lower drug loadings (10 and 20%), discrete drug rich domains formed and were associated with polymer-driven release in which the drug and polymer released congruently, while at higher drug loadings (30% and 40%), con nuous drug-rich domains were formed at the matrix surface that drama cally inhibited release of both the drug and polymer. Fluorescence imaging with the same marker molecules has been employed to characterize phase separa on of an expanding array of APIs and polymers in ASDs, with efforts made to iden fy the physicochemical proper es driving phase separa on behavior and associated altera ons in drug and polymer dissolu on .