Palynomorph Darkness Index (PDI) is proposed as a new thermal maturity indicator. PDI is calculated from measurement of the red, green and blue (RGB) intensities of light transmitted through palynomorphs, using standard palynological microscopes and digital cameras. PDIs determined by different microscope and camera combinations show excellent correlation, suggesting that the method is largely platform-independent, though calibration is required, preferably using photographic filters as standards. Investigation of PDI from experimentally heated Tasmanites reveals a progressive increase with increasing temperature, suggesting that the technique is applicable through a broad temperature range encompassing the whole of the oil window and at least part of the zone of dry gas generation. Potential applications of this inexpensive method include the estimation of thermal maturity of rocks deficient in vitrinite, such as the Lower Palaeozoic rocks of Arabia and the Upper Devonian black shales of North America.