A global picture for the structural evolution in the relaxor-ferroelectric solid solution (PbSc1/2Nb1/2O3)1-x-(PbTiO3)x is proposed. Thanks to x-ray profile analysis and Rietveld neutron powder refinement, a monoclinic phase has been evidenced in the morphotropic region (i.e., x≈0.43). This lower-symmetry phase “bridges” the rhombohedral Ti-poor phase (x0.55), in a similar way as in PbMg1/3Nb2/3O3-PbTiO3 or Pb(Zn1/3Nb2/3)O3-PbTiO3. For weak titanium concentration, we observe a macroscopic rhombohedral state with local monoclinic symmetry resulting from the combination between Pb and Sc/Nb/Ti shifts along [001] and [111] directions, respectively. Cationic competition with Ti doping increases the coherence length of this short-range monoclinic phase, which becomes long range in the morphotropic region. This intermediate monoclinic phase is in complete agreement with our first-principles calculations which predict Pm or Cm space groups. It has been shown that these ones are very close to each other in the free-energy space, and a minor change of atomic distribution and/or a slight modification in composition or in stoichiometry is enough to alter the space group of the monoclinic ground state. Finally, in the Ti-rich region, the monoclinic ground state is destroyed in favor of a tetragonal phase. © 2003 The American Physical Society.