The light-induced transient grating technique is combined to study photoelectrical properties of porous silicon sublayers with different structures in an optical way. This technique based on the measurements of a diffracted beam in the dark field ensures its sensitivity is higher than the techniques of differential absorption used for porous Si studies. Carrier diffusion and light absorption coefficients at 1.06 mu m of the lower layer of porous Si structures have been found similar to those in crystalline substrate. The free-standing films at high injection levels indicated fast non-equilibrium carrier recombination with a subnanosecond time.