The pressure dependence of the order-to-disorder transition in a polystyrene/polyisoprene (PS/PI) (M = 17000) and a polystyrene/poly(methylphenylsiloxane) (PS/PMPS) (M = 29600) diblock copolymer of symmetric composition has been studied using small-angle X-ray scattering. The order-disorder transition is investigated with a temperature resolution of 0.1 K. The transition can be resolved in a two-step process with a roughening of the interface preceding the breakdown of the lamellar order. For both systems a pressure-induced ordering transition is observed at constant temperature. The application of small pressures leads to a decrease of the transition temperature for both systems. High pressure shows the opposite effect. The minimum transition temperature for the PS/PI is located around p = 8 bar.