The nature of order-disorder transitions in cross-linked diblock copolymer melts was studied using small-angle X-ray scattering (SAXS) and birefringence measurements. Experiments were conducted on cross-linked polystyrene-block-polyisoprene copolymer samples wherein the polyisoprene block was selectively cross-linked at a temperature well above the order-disorder transition temperature of the pure block copolymer. We find a reversible transition from a disordered gel to an ordered gel when the cross-linking density is below a critical value. The rheological properties of disordered block copolymer gels are very different from that of cross-linked homopolymer gels.