Propylene glycol is generally used as inactive solvent for water-insoluble drugs in liquid form for administration orally and intravenously. The phase behaviour of these binary liquid mixtures is not known very well and this knowledge is desirable in the search for better drug design. In order to know the phase behavior, dielectric relaxation (10 -3 Hz -2 MHz) and differential scanning calorimetry (DSC) measurements have been performed on binary liquid mixtures of propylene glycol with four drugs namely acetaminophen, methocarbamol, guaifenesin, and mephenesin. The glass transition temperature region is examined critically for a step like change in DSC scan at the heating rate of 10 K/min. The DSC scans are found to be unusually broad in the glass transition region, especially for intermediate concentrations. This together with the dielectric spectroscopic results, which revealed two liquid like processes, each following Vogel-Fulcher-Tammanns temperature dependence at all concentrations, point to microheterogeneity in these mixtures. The two processes are attributed to the separated liquid phases rich in pharmaceutical and propylene glycol respectively.Pha rm a c e u tic a An a ly t ic a A cta