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Buckwheat, Fagopyrum esculentum, is an herb belonging to the family
Polygonaceae and is not, as popularly believed, a cereal grain.  Everything I
have ever read about it has stated that it contains no gluten.  Perhaps you
were looking at a product such as a buckwheat pancake mix as they often
combine buckwheat flour with a gluten containing flour.  I would be very
surprised if a box of pure buckwheat said it contains gluten.