WHEN a material absorbs a photon, a fraction of the energy may be transformed into heat. A measurement of photothermal heating as a function of wavelength can provide an absorption spectrum of the material. We have recently1,2 developed a micromechanical sensor capable of detecting heat changes of the order of picojoules (10-12 J). The instrument incorporates a bilayer cantilever of micrometre dimensions which bends in response to heating. Here we show that this device can be used for photothermal spectroscopy with a power sensitivity of 100 pW—two orders of magnitude better than the sensitivity of conventional photothermal deflection spectroscopy3. The small size of the sensor allows picogram quanti-ties of material to be studied, opening up the possibility of spectro-scopic studies on individual cells and bacteria. Being based on silicon technology, the sensor should be compatible with micro-electronic circuitry.