Process design requires knowledge of the physical and chemical properties of the fluids being handled. The measurement of a range of properties of two pyrolysis liquids is described. The pyrolysis liquid properties reported are: density, viscosity, refractive index, thermal capacity, thermal conductivity, and their variation with temperature, time, exposure to air/oxygen and history. The oils follow conventional behaviour for most properties except that there is marked differences in viscosity behaviour above around 50°C compared to below 50°C. There is a noticeable effect from exposure to oxygen/air which increases viscosity. This may be due to removal of volatiles, removal of water or reaction. There is clear evidence that some reaction occurs even under ambient conditions.