Femtosecond degenerate four-wave mixing (fs DFWM) experiments are applied to study the rotational coherences of perdeutero-benzene (C6D6) and pyridine (C5H5N). These systems are investigated at 20-40 K in a collision-free seeded supersonic jet and the spectra are compared to those obtained in a gas cell (298 K). The differences between the non-rigid, symmetric oblate C6D6 and the slightly asymmetric oblate C5H5N are discussed. From the simulation and fitting, rotational constants are obtained in good agreement with literature data. For the first time, a A DFWM spectrum (including asymmetry transients) of a non-rigid, asymmetric rotor molecule (C5H5N) was successfully fitted. (C) 2002 Elsevier Science B.V. All rights reserved.