This paper is an experimental study of the crystallographic circumstances governing the formation of brine drainage channels in sea ice. The study shows that brine channels always form in the grain broundaries toward which the ice platelets grow from both sides and that they do not form at the boundaries from which the platelets slant away. These observations imply that brine expulsion and gravity drainage are the major contributors to sea ice desalination. ¿ American Geophysical Union 1987