The early prediction of drug-excipient incompatibility is vital in the pharmaceutical industry to avoid costly material wastage and time delays. We report here on the use of high sensitivity differential scanning calorimetry (HSDSC) to examine the compatibility between an experimental drug (Drug A) and common pharmaceutical excipients. Short-term HSDSC experiments (up to 25h) indicated that Drug A was stable in the presence of moisture and was compatible with both lactose monohydrate and magnesium stearate in the dry state, but showed degradation in the presence of magnesium stearate and water in combination. These results agreed with conventional stability studies, in which extensive degradation was observed in the Drug A-magnesium stearate system after storage at 40 degrees C/75% RH for 4 weeks but not under other conditions. These results indicate that HSDSC may be used to examine the compatibility of experimental drugs with conventional excipients and, in particular, illustrate the importance of incorporating humidity as an experimental variable in order to fully establish the stability profile of the material under test.