The IIB supersymmetry algebra in d=10 can be extended including 10-forms. One of these is the RR 10-form, which takes part in the tadpole cancellation mechanism of the type-I string. D9-branes are electrically charged with respect to this 10-form. IIB supergravity possesses a global SL(2,R) symmetry, which the quantum theory breaks to SL(2,Z). Even if 10-forms have no dynamics, since their field-strength vanishes identically, it is possible to study their behaviour under SL(2,Z) transformations. We determine all the possible independent 10-forms as representations of SL(2,Z), and in particular we show in which multiplet the RR 10-form lies. Finally, we perform the same analysis for the IIA supersymmetry algebra, and interpret the results in terms of T-duality.