A pilot study of the optical properties of graptolite skeletal materials shows progressive changes of reflectance, bireflectance, refractive index and absorptive index, produced by temperature changes related to depth of burial. Samples of Lower Ordovician to Upper Silurian graptolites were selected from across Canada, from locations where the conodont alteration indices of adjacent conodont collections provide a control index of temperature. The reaction to temperature of graptolite skeletal material appears to be similar to that of bitumen, but somewhat different to that of the vitrinite and liptinite maceral components of coal. Maximum reflectance in oil and bireflectance appear to be the characteristics most diagnostic of maximum temperature, and both show progressively higher rates of increase at higher temperatures. These factors show potential for more sensitive discrimination of temperatures at high levels of conodont alteration indices. The optical properties of graptolites may perhaps provide a useful tool for unravelling the post-depositional histories of Lower Palaeozoic rocks in deformed belts.