The glass transition temperature of whey proteins concentrate (WPC)/hydroxypropyl methylcellulose (HPMC) co-dried mixtures with different degrees of phase separation and morphologies were determined by differential scanning calorimetry. To this end the phase separation of aqueous mixtures of WPC (12 wt% or 20 wt%) and HPMC (2 wt% or 3 wt%) at pH 5 or 6, was arrested at different times before freeze-drying. Confocal microscopy allowed to characterize the morphology of phase separation. Co-dried mixture from quenched phase-separated systems exhibited different numbers of Tgs, according to the degree of phase separation. Two Tgs were observed in the fully phase-separated systems. A single Tg was detected during the first stages of phase separation (i.e. below a 50% of phase separation). It is proposed to ascribe the observed single Tg to the predominance of the extremely large mixed protein/polysaccharide interface present, that would dominate the mobility of the whole system because acting as a network for the entanglement between the protein-rich and the polysaccharide-rich phases. WPC (12 wt%)/HPMC (2 wt%) co-dried mixture at pH 5, with a degree of phase separation above 50%, exhibited three Tgs which were related respectively to the mixed interface, protein-rich phase and polysaccharide-rich phase. The non-phase-separated WPC (6 wt%)/HPMC (1 wt%) co-dried mixture also showed a single Tg with a reasonable agreement to the predicted value by a theoretical model. (C) 2009 Elsevier B.V. All rights reserved.