A transient metastable rotator phase occurs on crystallization of n-alkanes into their triclinic crystalline phase from the supercooled melt. This was directly observed using time resolved X-ray scattering. The observed crystallization temperature is shown to be determined by the thermodynamic stability of the transient phase with respect to the liquid. The crystallization kinetics of the homologous series of n-alkanes was measured and explained in terms of a crossover from stability to "long-lived" metastability to transient metastability. This work highlights the importance of transient metastable phases in crystallization.