Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool. The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C2, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame. Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields. © 2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS.