Photo-thermal deflection technique is used to study the nonradiative recombination process in laser structures Ga_0.7In_0.3As_0.23Sb_0.77 and Al_0.7Ga_0.3As_0.08Sb_0.92 grown by molecular beam epitaxy on GaSb substrate. A two layer theoretical model has been developed, taking into account both thermal and electronic contributions in the photothermal signal; the coincidence between experimental curves giving the normalized amplitude and phase variations versus square root modulation frequency to the corresponding theoretical ones permits to determine non-radiative lifetime, electronic diffusivity, and surface and interface recombination velocities.