  We analyze transcendentality for certain terms that arise in multiloop
amplitudes in the low momentum expansion of the four graviton amplitude in type
IIB string theory in ten dimensions, based on the constraints of supersymmetry
and S--duality. This leads to several contributions that violate
transcendentality beyond one loop at all orders in the low momentum expansion.
We also perform a similar analysis for the five graviton amplitude, obtaining
contributions that involve single--valued multiple zeta values beyond tree
level.
