  Explicit expressions for one-loop five supergraviton scattering amplitudes in
both type II superstring theories are determined by making use of the pure
spinor formalism. The type IIB amplitude can be expressed in terms of a
doubling of ten-dimensional super Yang--Mills tree amplitude, while the type
IIA amplitude has additional pieces that cannot be expressed in that manner. We
evaluate the coefficients of terms in the analytic part of the low energy
expansion of the amplitude, which correspond to a series of terms in an
effective action of the schematic form D^{2k}R^5 for 0\le k \le 5 (where R is
the Riemann curvature). Comparison with earlier analyses of the tree amplitudes
and of the four-particle one-loop amplitude leads to an interesting extension
of the action of SL(2,Z) S-duality on the moduli-dependent coefficients in the
type IIB theory. We also investigate closed-string five-particle amplitudes
that violate conservation of the U(1) R-symmetry charge -- processes that are
forbidden in supergravity. The coefficients of their low energy expansion are
shown to agree with S-duality systematics. A less detailed analysis is also
given of the six-point function, resulting in the vanishing of the analytic
parts of the R^6 and D^4 R^6 interactions in the ten-dimensional effective
action, but not in lower dimensions.
