Leibler's random phase approximation for block copolymers, modified so as to be applicable to linear multiblock copolymers, has been quantitatively compared to data from three linear ABC/ACB triblock copolymer melts: poly(cyclohexylethylene-b-ethylene-b-ethylethylene) (CEEE)/CEEE, poly(styrene-b-isoprene-b-ethylene oxide) (SIO)/ISO (Macromolecules 2001, 34, 6994,-7008;Macromolecules 2002, 35, 7007-7017), and poly(styrene-b-isoprene-b-dimethylsiloxane) (SID)/ISD (Macromolecules 2002, 35, 3189-3197). The RPA calculation provides the mean-field static structure factor for a disordered block copolymer melt, which can be used to anticipate the scattering behavior and spinodal stability limit temperature (Ts); in the context of mean-field theory, the spinodal should lie near the order-disorder-transition temperature (TODT). We find that the RPA spinodal temperature semiquantitatively matches the magnitude and temperature dependence of the ODT as a function of molecular weight (in CEEE/CEEE) and composition (SIO/ISO). Furthermore, the structure factor also reproduces scattering phenomena in ABC triblock copolymers (ISD/SID) such as the two-peak profile observed in disordered ISD with X-ray scattering. The results show that the RPA is a useful tool in the design of multiblock copolymers without reliance on existing experimental data or cumbersome numerical self-consistent field calculations.