Critical behaviour of blend solutions of polystyrene (M(w) = 3.55 x 10(5)/poly(methyl methacrylate) (M(w) = 3.27 x 10(5)) in d6-benzene was investigated by static light scattering. The phase diagram (spinodal, binodal and cloud points) was determined at a fixed total polymer concentration. The system had the lower critical solution temperature. The inverse isothermal osmotic compressibility (partial derivative pi/partial derivative c1)T and correlation length xi of concentration fluctuations were determined as functions of the reduced temperature epsilon = T(s)/T-1 near the stability limit, where T is the absolute temperature and T(s) is the spinodal temperature. Both (partial derivative pi/partial derivative c1)T and xi were described by single exponents over the investigated temperature range. The critical exponents gamma and nu for (parital derivative pi/partial derivative c1)T and xi were 1.23 and 0.63, respectively. They were very close to the three-dimensional Ising exponents and obviously different from the mean-field or Fisher’s renormalized Ising exponents, as theoretically predicted by Broseta et al.