Assessing photostability (particularly of pharmaceuticals) is of growing importance, but hampered by a lack of reliable, rapid experimental testing protocols and instrumentation. In particular, most approaches require irradiation of the sample separately from the analytical measurement, which increases both experimental complexity and the number of assumptions that must be made when calculating stability. One technique that may obviate this is photocalorimetry, principally because the reporter of change (heat) is measured directly as a sample is irradiated. Although not a new idea, the design challenges of photocalorimeters are complex, primarily because light power is being introduced to the calorimeter which can thus both saturate the amplifiers and swamp the response of the sample. Careful instrument design is thus paramount. The aim of this work was to develop a robust, compact, and easy to use photocalorimeter with the immediate focus of developing photostability assays for pharmaceuticals. The final instrument design, arrived at through a series of iterative design modifications, is based on a twin differential heat-conduction principle and achieves an average base line deflection of -0.04+/-0.11 microW with light irradiating the sample cell. The performance capabilities of the instrument were demonstrated using a model system; the photodegradation of 2-nitrobenzaldehyde in solution.