Got two sets of merino stuff, one does have a very small amount of polyester the other 100% merino. The 100% one can get a bit stretchy, the other holds its shape well. I get my merino socks from Marks and Sparks. They are great, cool in the summer and warm in the winter and thin so it is easy to wear two pairs when really cold but rarely need to as I use sheepskin inner soles. merino is great stuff in my view.