  We study maximally symmetric cosmological solutions of type II supersymmetric
strings in the presence of the exact quartic curvature corrections to the
lowest order effective action, including loop and D-instanton effects. We find
that, unlike the case of type IIA theories, de Sitter solutions exist for type
IIB superstrings, a conclusion that remains valid when higher-curvature
corrections are included on the basis of SL(2,Z) invariance.
