We show that the M-theory/IIA and IIA/IIB superstring dualities togeth
er with the diffeomorphism invariance of the underlying theories requi
re the presence of certain p-brane bound states in ITA and IIB superst
ring theories preserving 1/2 of the space-time supersymmetry. We then
confirm the existence of TIA and IIB supergravity solutions having the
appropriate p-brane bound states interpretation. (C) 1998 Elsevier Sc
ience B.V.