Rayon is a semi-natural fiber - it is essentially re-processed cotton. Our rayon braid has more sheen and the colors available are a more muted palette. Polyester is an entirely synthetic fiber. It takes color beautifully, so our polyester braid is available in brighter colors. The sheen is slightly duller than that of the rayon. The entire line of Bead Smith Soutache was designed by a textile jewelry artist for textile jewelry artists so tremendous attention was paid to consistency of width, thickness, and drape so that the rayon and polyester braid can be used together successfully.