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The kinetics of phase separation in thin polymer blend films of polystyrene and polybutadiene on a silicon substrate is examined by optical microscopy of the free film boundary. Our observations on 1000 and 200 Å films are consistent with a crossover from three-to two-dimensional spinodal decomposition kinetics in the (off-critical) viscous hydrodynamic regime. In this stage of phase separation the exponent n, characterizing the scale R (t)∼ t n of the coarsening pattern, is predicted to change from 1 to a value near 0.46 upon lowering the dimensionality.