Spinodal decomposition in multicomponent mixtures of two homopolymers and a block copolymer was studied by a combination of neutron and light scattering experiments. Mixtures of nearly monodisperse polyolefins-polymethylbutylene (M(w) = 1.7 x 10(5) gm/mol), polyethylbutylene (M(w) = 2.2 x 10(5) gm/mol), and a symmetric polymethylbutylene-block-polyethylbutylene (M(w) = 4.6 x 10(4) gm/mol) were studied, following relatively deep quenches into the spinodal region (-)chi/chi(s) ranged from 1.7 to 2.4 (chi is the Flory-Huggins interaction parameter at the experimental temperature and chi(s) is the Flory-Huggins interaction parameter at the spinodal temperature). The ratio of homopolymer volume fractions was kept constant at unity, and the block copolymer volume fraction was varied from 0.0 to 0.2. The evolution of structure was followed over five decades of real time-1 min to 1 month. During this time, the characteristic length scale of the phase separated structure increased from 10(-1) to 10 mu m. The early stages of spinodal decomposition, captured by time-resolved neutron scattering, were compared with theoretical predictions based on the random phase approximation (RPA). Qualitative agreement was obtained. The intermediate and late stages, studied by light scattering, followed classic signatures of binary spinodal decomposition. Experimental evidence indicates that the block copolymer is uniformly distributed throughout the sample during all stages of the decomposition.