A dual-beam photothermal reflection based system capable to measure thermal diffusivities of optically dense liquids has been designed and implemented. The large optical absorption coefficient of these liquids inhibits the possibility to use conventional transmission instruments for direct thermal diffusivity measurements. To overcome this problem, a front heating front detection photothermal reflection system has been proposed. This method expands the range of application and simplifies the experimental procedure of traditional photothermal methods, allowing precise measurement of thermal diffusivity of a variety of liquids. Measurements of the change in thermal diffusivity with the concentration of asphaltene in toluene solutions are described to test the applicability of this technique for reliable measurements of thermal diffusivities of optically dense liquids. © 2010 American Institute of Physics. doi:10.1063/1.3297900