I will discuss the mathematical structure of tree level amplitudes among massless superstring states. String corrections to these amplitudes take a compact and elegant form once the contributions from different classes of multiple zeta values (MZVs) are disentangled. The idea is to lift MZVs to their motivic versions endowed with a Hopf algebra structure: It induces an isomorphism which casts the amplitudes into a very symmetric form and represents the generalization of the symbol of a transcendental function. I will also comment on generalizations to loop amplitudes.