We report the first neutron scattering study of a condensed-phase transformation in an effusion cell at high temperatures. Neutron powder diffraction patterns of gallium sulfide in a quartz effusion cell were measured from room temperature to 1260 K to seek a phase transformation at 1243 K implied by mass-spectrometric and vapor-pressure data and predicted with a thermodynamic theory of condensed-phase transformations in binary systems. Diffraction patterns in the range of 1230-1260 K revealed a phase transformation from monoclinic Ga2S3 to a high-temperature phase, which was indexed from neutron diffraction data at 1260 K as hexagonal with unit cell parameters a = 3.74 +/- 0.20 a and c = 6.14 +/- 2.69 Angstrom. Diffuse scattering observed in the diffraction pattern of the hexagonal, high-temperature phase suggested disordering in Ga2S3 and correlation in gallium positions on the length scale of their sublattice spacing in the monoclinic phase. The time scale of the experiment, > 1 h, prevented a determination of the order of the transformation. This application of time-of-flight neutron powder diffraction to samples in effusion cells has introduced a new high-temperature technique for structural studies of phase equilibria of materials under effusion conditions.