Barley is not gluten-free as it is one of three grains that are primarily gluten-based, according to About.com. Barley seeds are known to contain gluten, but the presence of gluten in barley grass used in vitamins is not completely clear. Most food items are created with the barley seeds.
The Mayo Clinic agrees that barley contains gluten, as do wheat and rye. Therefore, those with celiac disease or anyone following a gluten-free diet should avoid malt, malt flavoring and malt vinegar. Specific food products to avoid include beers, barley flour and soups containing barley. Barley malt may also be present as a sweetener in cereal or candy.