The efficiency of the non resonant two photon (NRTP) isomerizations of stilbenes and 1,4 diphenyl 1,3 butadienes was found to depend strongly on the wavelength of the laser pulses. The results not only confirm the 2 1Ag nature of the excited states responsible for the cis trans isomerizations but also demonstrate that the action spectra of the NRTP reactions correspond to the two photon allowed absorption bands.