An in situ monitoring furnace was constructed with side windows to perform partial pressure measurements by optical absorption and visual observation of the growing crystal. A fused silica growth ampoule with a 4.5 cm long square tube between the source and the seed was prepared for the optical absorption measurements. A ZnSe crystal was grown by the seeded physical vapor transport (PVT) technique in a horizontal configuration. The growth temperature was 1120 degrees C and the furnace translation rate was 3 mm/day. Partial pressures of Se-2, P-Se2, at three locations along the length of the growth ampoule were measured at 90 min intervals during the growth process. The measured PSe2 were in the range of 2.0-6.5 x 10(-3) atm. The P-Se2 results indicate that the partial pressure profile was inconsistent with the predictions of the one-dimensional diffusion mass transport model and that the source composition shifted towards Se-rich during the run, i.e. the grown crystal was more Zn-rich than the source. The visual observation showed that the seed crystal first etched back, with greater thermal etching occurring along the edges of the seed crystal. Once the growth started, the crystal grew in a predominately contactless mode and facets were evident during growth. The crystal did not grow symmetrically, which is believed to be due to the unintentional asymmetry of the radial thermal profile in the furnace. Published by Elsevier Science B.V.