A dual-beam mode-mismatched configuration of the Thermal Lens (TL) technique was applied to characterize biodiesel, diesel and diesel/biodiesel blends. The excitation was performed using a tunable laser in the near-near infrared region in order to assess overtones and combination bands of the vibration modes. The thermo-optical properties of the fuels were also determined. The results showed that the biodiesel contents in diesel/biodiesel blends can be monitored with this technique, by excitation the third overtone of the C-H (4 nu(C-H)) bond stretching vibration at 916 nm. Both the amplitude of the photothermal signal and thermal diffusivity are important parameters to quantify the biodiesel content in diesel/biodiesel blends.