We present the first detailed analysis of a transient grating (TG) experiment on a supercooled molecular liquid, m-toluidine, from 330 K (1.75 Tg) to 190 K (1.01 Tg) based on the theoretical model proposed in Pick et al (2004 Eur. Phys. J. B 39 169). This hydrodynamic model of the TG response function yields a complete description of the experimental data and a characterization, over a wide dynamical range, of the different physical phenomena giving rise to the signals, including the rotation–translation coupling process. Using a disentanglement procedure of the isotropic from the anisotropic part of the TG response and a careful fitting analysis, we obtain detailed information on the rotation–translation coupling function. We also extract the structural relaxation times related to the 'longitudinal' viscosity over almost 10 decades in time. The value of some other parameters and information on their thermal behaviour is also reported.