Per REI's product page:
"Expedition-weight merino wool/Capilene® socks keep hikers' and climbers' feet dry and warm in luxurious softness
--Capilene polyester wicks moisture away from skin for dry comfort while soft, no-itch merino wool provides natural warmth
--Anatomically-shaped toe boxes, thin toe seams and arch band reduce bulk for a snug, blister-resistant fit
--Cuffs at top help socks stay put
--Made from 65% merino wool/20% Capilene polyester with 15% stretch nylon for durability"