The origin of optical diffraction in holographic polymer-dispersed liquid crystal (H-PDLC) transmission gratings was investigated by optical two beam-coupling analysis based on the linear phase-shift technique. Gratings with the pitch of 1 micrometer and the thickness of 50 micrometers were fabricated from a UV curable mixture of commercially available constituents. Photopolymerization in the interference field of two laser beams produces not only a periodic variation of the refractive index, but also a periodic modulation of optical extinction due to light scattering. Both of them contribute to the diffraction efficiency of the gratings. The magnitudes and relative phases of the two contributions were measured as a function of a recording time of the grating and as a function of an applied external electric field. During the initial stage of the grating formation phase modulation is predominant, while at longer exposures both contributions have the same order of magnitude. They are phase shifted with respect to each other for around ?/2. Application of an external electric field causes a strong decrease of the amplitude modulation, while phase modulation is much less perturbed.