The critical behavior of six polyisoprene/poly(ethylene-propylene) binary blends has been examined, primarily by dynamic light scattering. Specifically, the decay rate (Γ) of fluctuations in composition has been measured as a function of scattering vector (q) and temperature, above the stability limit. The results are interpreted in the context of mode-coupling corrections to the dynamics. Theoretically predicted crossovers between mean-field and nonclassical regimes, and between mode-coupled and mode-decoupled dynamics, are clearly seen. The mean-field to nonclassical crossover, or Ginzburg criterion, is localized, and the mode-coupled to mode-decoupled transition in the dynamics occurs well into the static mean-field regime. In addition, geometric crossovers between Γ∼q2 ("diffusive") and Γ ∼ q3 or Γ ∼ q4 ("critical nondiffusive") regimes are in quantitative agreement with theory. The applicability of the Kawasaki-Stokes relationship between Γ and the dynamic correlation length is also demonstrated.