<p>Massless type IIB superstring amplitudes are organized according to the number of external states and their U(1) charge under the R-symmetry of type IIB supergravity. In this work, we analyze the low-energy expansion of one-loop five-point amplitudes in all charge sectors, focusing on the representative processes involving five gravitons and four gravitons with one dilaton. We compute the one-loop contributions to the moduli-dependent couplings in the type IIB effective action up to the <i>D</i>¹²<i>R</i>⁵ and <i>D</i>¹⁴<i>ϕR</i>⁴ interactions. The results are consistent with <i>S</i>-duality constraints in every charge sector and exhibit rich arithmetic structure, including single-valued multiple zeta values, affine linear combinations of logarithmic derivatives of the Riemann zeta function at odd integers, and a new constant of currently unknown nature.</p>