The phenomenon of phase separation by spinodal decomposition was studied for polymer blends made by compositional quenching. The modified Cahn-Hilliard theory of phase separation was extended to include hydrodynamics, with a volumetric body force, due to concentration gradients, that induced convective flows. This force influenced the morphology and the growth rate of the average domain size. Unlike the conventional treatment of flows driven by surface tension, the velocity and pressure fields were treated as continuous functions of spatial position. Numerical solutions for the phase separation in a binary mixture were obtained for a three-dimensional system with periodic boundary conditions. For near critical quenches with similar volume fractions, for the two components, cocontinuity was destroyed by the hydrodynamics, giving discrete domains. The breakup in interconnectivity is believed to be a universal phenomenon. The domain growth rate followed a power law, r --> tau(n). The growth exponent depended on the dimensionless viscosity group, zeta = (R(g)T/nu(s)) (kappam/muD(AB)) and ranged from n = 0.32 +/- 0.006 for zeta = 0 (no hydrodynamic effects) to n approximately 1 for zeta = 1. For off-critical quenches in which a dispersed phase would be formed by diffusion alone, the scaling exponent showed little enhancement. The simulations accurately predicted the particle size formed in the early stages of spinodal decomposition.