The phase shift between the pattern of light onto a photorefractive crystal and the resulting hologram at the very beginning of the recording process in two-wave mixing is analyzed and measured as a function of the applied electric field. These data allow one to compute the diffusion length of photoexcited charge carriers and to evaluate the actual electric field inside the crystal. A diffusion length of 0.14 &mgr;m is measured in a nominally undoped photorefractiveBi12TiO20crystal using a 532 nm wavelength laser illumination, in agreement with results obtained from other methods. ©1997 American Institute of Physics.