The first observation of ordered microphase structures in poly(oxyethylene)-poly(oxypropylene) diblock copolymers melts is reported. Two diblock copolymers were synthesised by anionic polymerisation, i.e. E130P58 and E107P69 where E represents an oxyethylene unit, OCH2CH2, and P an oxypropylene unit, OCH2CH(CH3). The volume fractions of the two components were in the range 0.5-0.6. Order in the melt phase was confirmed by small angle X-ray scattering, and order-to-disorder transitions (ODT) were located at 106 degrees C (E130P58) and 114 degrees C (E107P69). In a related experiment, a triblock copolymer with perdeuterated P blocks, E(33)dP(42)E(33), was synthesised and studied in the melt phase by small-angle neutron scattering. This allowed determination of the temperature dependence of the Flory-Huggins interaction parameter for the poly(oxyethylene)-poly(oxypropylene) system, i.e. chi = 20.2/T + 0.0221.