Nanothermal analysis (NTA) supported by atomic force microscopy imaging has been used to study the changes that occur at the surfaces of solid dispersions of the drug felodipine and the water soluble polymer, polyvinylpyrrolidone (PVP) on exposure to standard pharmaceutical environmental stress conditions. Exposure to relative humidities above 75% (at 40 °C) was sufficient to achieve phase separation of the drug and polymer into areas which displayed a glass transition temperature consistent with pure drug and polymer over a period of a few days. Higher values of humidity at 25 °C (e.g. 95%RH) were also sufficient to cause such phase separation within a day. Extended studies of up to two months showed an eventual crystallization of the drug. NTA is shown to be effective at the early detection of instabilities in solid dispersions and the quantifiable identification of the relative composition of phase separated domains based upon their glass transition temperatures. The combined nanoscale analytical approach employed here is able to systematically study the influence of storage conditions and different drug loadings and to evaluate physical stability as a function of environmental conditions.