Binary blends of statistical polybutadiene copolymers of different vinyl content were explored by small-angle neutron scattering. These samples represent the simplest class of statistical copolymer mixtures. In spite of this simplicity, strong changes of phase behavior were observed with varying vinyl content, molar volume, and deuteration; the temperatures of phase separation are found between -230 degreesC and more than 200 degreesC and phase separation can even reverse from an enthalpically driven one at low temperatures (UCST) to an entropically driven one at high temperatures (LCST). The experimentally determined entropic and enthalpic terms of the Flory-Huggins parameter are in excellent agreement with lattice cluster theory calculations by Dudowicz and Freed.