Ease of removal of the husk of oats from its enclosed kernel, hullability, affects the efficiency with which oats are milled. Hullability of a range of oat varieties was examined using a mechanical dehuller from autumn-sown trials in Northern Ireland, 1996–98, and in the Republic of Ireland, 1997–98. Varieties differed greatly in hullability, the spring variety, Barra, having good hullability while the winter variety, Gerald, had poor hullability. Differences between the varieties were largely consistent from site to site and from year to year despite large differences in the amounts of grain remaining unhulled from trial to trial. Although secondary grains were easier to dehull than primary grains, differences in the structure of the grain populations did not explain variation in hullability of the varieties. This study highlights the need for an investigation of the effects of agronomic factors on hullability.