It is more natural than polyester. It is a fibre made from reconstituted cellulose, usually from beech trees. It is a lot lighter,softer and more"breathable" than polyester. They are very comfortable to wear, I love garments made from modal and prefer them to polyester.Is modal not just Posh polyester? Have no idea but on the Carol Hochman Midnight collection (notice no XXL or up here) they were saying how incredible modal was?