Two‐beam coupling gain coefficients of transmission and reflection gratings inBaTiO3:Ce crystals versus wavelength between 570 and 680 nm have been measured. The former decreases more quickly than predicted by theory. The latter was found to vary significantly with wavelength, while theory shows that it should vary little. In order to explain the experimental results we assumed that the effective carrier number densityNeffdepends on wavelength. By calculatingNefffrom the measured results we found thatNeffis approximately a linearly decreasing function of wavelength. The slopes and intercepts for the linear functions are given for the two crystals studied. ©1995 American Institute of Physics.