A model set of six homogeneous (disordered) 1,4-polybutadiene-1,2-polybutadiene diblock copolymers containing perdeuterated 1,4-polybutadiene blocks have been examined by small-angle neutron scattering (SANS). Each of the SANS spectra exhibits a peak in the scattering intensity I(q*) at wave vector q*, which diverges upon approaching the microphase separation transition (MST). These findings are in close agreement with the MST theory of Leibler, which predicts that I(q*) should depend on the product of the degree of polymerization N and interaction parameter x; this provides a new, sensitive method of determining X for polymer pairs. The theory fails to account for “domain” scattering, which results in an unpredicted increase in the level of q < q* scattering as xN→(xN)MSTin samples containing 18-25% 1,4-polybutadiene. This investigation corroborates our previous findings concerning the rheological properties of block copolymers near the MST.