[en] It is shown that the absolute phase of a weak reflection can be obtained from an experimental integrated intensity measurement by taking into account multiple-beam effects. The absolute structure factor of the basis-forbidden 622 reflection in Ge has been measured. The contribution from weakly excited reflections whose phases are known acts as a reference wave for the reflection in question. It is further shown that, even in the limit of weak excitation of multiple beams, the intensity distribution in reciprocal space is determined by dynamical, not kinematical, theory. The remarkable result is demonstrated that several hundred beams are required to fully calculate X-ray multiple-beam effects, and an efficient approximation is presented for multiple-beam dynamical calculations with weak reflections. The intensity of weak multiple-beam effects, at least in the diamond structure, is roughly proportional to Z⁴, where Z is the atomic number. (orig.)