Recently, there has been significant interest in measuring the glass transition temperature (T-g) of thin polymer films floated atop liquid substrates. However, such films still have intrinsically asymmetric interfaces, that is, a free surface and a liquid-polymer interface. In an effort to analyze the influence of different liquids on the T-g of confined polymers in which there is no interfacial asymmetry, a colloidal suspension of polystyrene (PS) nanoparticles (NPs) was employed. The T(g)s of PS NPs suspended in either glycerol or an ionic liquid were characterized using differential scanning calorimetry. Nanoparticles suspended in an ionic liquid showed an invariance of T-g with confinement, that is, decreasing diameter. In contrast, nanoparticles suspended in glycerol showed a slight decrease in T-g with confinement. The dependence of NP T-g on the nature of the surrounding liquid exhibited a positive correlation with the interfacial energy of the liquid-PS interface and no correlation with interfacial softness, as measured by viscosity. A comparison of the results with thin films supported by liquid or solid substrates revealed a nontrivial interplay between interfacial softness and interfacial interactions on the T-g of confined PS. (C) 2016 Wiley Periodicals, Inc.