Small-angle x-ray scattering (SAXS) from single-phase melts and solutions of polystyrene-block-polyisoprene (SI) having various degrees of polymerization N and compositions f were analyzed on the basis of Landau-type mean-field theory developed by Leibler. The effective Flory interaction parameter chi(eff) per segment in melts and in semidilute Theta solutions were determined from the mean-held analysis of the SAXS profiles obtained at temperatures higher than T-MF, the crossover temperature above which the non-mean-field effects become less significant and the mean-field theory becomes a good approximation. It was found that chi(eff) thus determined depends on N, f, volume fraction of the SI in the solution phi(p) and the interaction parameter chi(0) in melt : chi(eff)=phi(p) chi(0) and chi(0) is a function of f and N; chi(0) at a given f is a decreasing function of N and that at a given N is approximately a quadratic function off with a minimum at f congruent to 1/2.