We have used dynamic oscillatory measurements to locate the temperature of the microphase separation transition (T-MST) for a series of nearly monodisperse styrene-isoprene diblock (SI) and triblock (SIS) copolymers, all containing 13 wt % styrene and ranging in molecular weight from 48K to 149K. The microphase-separated morphology in all cases is a disordered (liquidlike) arrangement of polystyrene spheres in a polyisoprene matrix, as determined by small-angle X-ray scattering. Values of T-MST were obtained from isochronal plots of the storage modulus versus temperature. The marked change in slope at the MST demonstrates that even materials with a disordered microdomain structure can experience significant property changes when the MST is traversed. The diblock and triblock T-MST values are consistent with a single chi(T) relation, indicating that targe-scale polymer architecture has little effect on chi.