Linear viscoelastic dynamic theological measurements were performed on symmetric diblock copolymers of deuterated polystyrene and poly(n-butyl methacrylate), denoted P(d-S-b-nBMA). Isothermal frequency sweep data were time-temperature superposed. A transition from a disordered to ordered morphology was found with increasing temperature as a function of molecular weight. P(d-S-b-nBMA) with molecular weights less than 6.8 x 10(4) were always disordered, as evidenced by the scaling of the storage and loss moduli with frequency at low frequencies, G’ proportional to omega(2) and G’ proportional to omega. Copolymers with molecular weights above 9.9 x 10(4) were always ordered, exhibiting very long relaxation times and appearing almost solidlike in the low-frequency limit. Mixtures of the 6.8 x 10(4) and 9.9 x 10(4) copolymers, however, showed a transition from fluid to solidlike behavior as the temperature was increased. This theological data is consistent with the existence of a lower critical ordering temperature, LCOT, where a transition from a disordered to ordered state occurs with increasing temperature.