We discuss first the small-angle scattering of neutrons by a homopolymer melt, when each chain is tagged at certain sites by deuteration. We then extend these considerations to block copolymers AB, assuming an AB interaction which is weakly repulsive. Starting from the homogeneous melt, we lower the temperature and determine first the locus of the spinodal instability towards microphase separation, and in particular the period of the incipient periodic structure. L. Leibler has recently constructed a Landau theory for the microphases, and a theoretical phase diagram (restricted to the vicinity of the melt) showing regions with lamellar, hexagonal or cubic structures. Finally we discuss the effect of a certain amount of disorder in the chemical sequence on the spinodal instability: in the most interesting case, where the disorder is the same in all chains, we find that disorder increases the spatial period and ultimately suppresses the trend toward microphases. These last considerations may be relevant for the design of certain nematic polymers.