We measure the optical two-beam coupling gain in three nominally undoped cubic n-type Bi12SiO20 crystals as a function of the beam frequency difference Ω and the beam intensity. The crystal geometry is chosen so that the beam coupling through the ordinary photorefractive effect is absent. From the dependence of the gain on Ω, the full complex polarizability difference αfe between a full and an empty deep trap at the wavelength of 515 nm is deduced to be (−1.8 − 4.6i ± 0.7 ± 1.1i) × 10−39 F m2 in SI units or (−1.6 − 4.1i ± 0.6 ± 1.0i) × 10−23 cm3 in Gaussian units. This suggests that the hole photoexcitation cross section σh is larger than that for an electron, σe. Our data are consistent with the electron and hole parameters deduced from extensive previous measurements (in one of the crystals) analyzed with the standard electron–hole competition equations. This consistency requires that the average density of full traps be at least 20 times larger than the average density NA of empty traps and that σh be (2.4 ± 0.8) × 10−17 cm2, while NA is (1.4 ± 0.4) × 1016 cm−3 and σe is less than −6 × 10−18 cm2. This is to our knowledge the first determination of these parameters in a sillenite crystal.