[en] We compute long-distance effects on the photon energy spectrum in inclusive radiative decays of B mesons using light-cone expansion and heavy-quark effective theory. We show that for sufficiently high photon energy, the leading nonperturbative QCD contribution is attributed to the distribution function. The distribution function is found to be universal in the sense that the same distribution function also encodes the leading nonperturbative contribution to inclusive semileptonic decays of B mesons at large momentum transfer. Some basic properties of the distribution function are deduced in QCD. Ways of extracting the distribution function directly from experiment and their implications are discussed. The theoretically clean methods for the determination of vertical stroke V_ts vertical stroke are described. (orig.)