The effective interaction parameters, (chi(F)) of the homologous series of deuterated polystyrene-block-poly(n-alkyl methacrylate) copolymers (dPS-b-PA(n)MA) from methyl (n = 1) to hexyl (n = 6) side groups have been investigated by small-angle neutron scattering and a Hartree (fluctuation correction) analysis. While chi(F) for dPS-b-PA(n)MA with n = 1 and 6 is a typical decreasing function of temperature, it is shown that chi(F) for n from 2 to 4 changes to a monotonic increase with temperature, revealing growing upward convexity. This action culminates in a maximum Of chi(F) for n = 5, which suggests an immiscibility loop-type behavior. Using the Hartree analysis for compressible diblock copolymers, the effect of directional interactions and the compressibility difference between the constituent blocks on chi(F) is discussed. The ordering transition temperatures, their pressure dependences, and chi(F) for the homologous block copolymer series are shown to be in good agreement with the experimental values.