A photocalorimeter was designed to analyse quantitatively the photostability of pharmaceuticals. Its application is demonstrated with reference to two solution phase test systems; the photodegradations of 2-nitrobenzaldehyde (2-NB) and nifedipine. Light emitting diode (LED) arrays were used to illuminate the sample and reference channels of the calorimeter. Five LEDs were used to create an array from 360 to 700 nm. A power supply system was constructed that zeroed the instrument by supplying a preset voltage to the sample side LEDs and varying that supplied to the reference LEDs until a zero calorimetric signal was obtained. The photodegradation of 2-NB was zero-order and varied in proportion to the input voltage supplied to the LED array. Analysis of the data (the rate of reaction was determined to be equal 1.04 × 10−6 mol dm−3 s−1 by pH titration) determined a reaction enthalpy of 5.0 ± 0.6 kJ mol−1. In the case of nifedipine, the LEDs in the array were operated individually in order to determine the causative wavelength of degradation. This was found to be 360 nm, in agreement with the literature.