The crystallization history of syndiotactic polystyrene is found to affect strongly its mechanical properties. Samples cooled slowly from the melt have much higher moduli than do samples quenched from the melt (to the amorphous state) and subsequently heated to induce crystallization just above T-g. The differences are not due to differences in crystallinity. Examination of the lamellar structure and micromechanical modelling of the two different micromorphologies which evolve, show that the most likely reasons for these differences are the constraint of the amorphous phase by the high-modulus lamellae, and the variation in phase contiguity. In slowly cooled samples, the crystalline phase is contiguous (mechanically), and the interlamellar amorphous material is more highly constrained than in the quenched-annealed samples.