A series of polystyrene-block-polyisoprene diblock copolymers having various degrees of polymerization N-n and composition f(PS) were studied as a function of temperature T and polymer concentration phi (p) by means of in sitn small-angle X-ray scattering methods. The results indicated that the characteristic length D as evaluated from D = 2 pi /q(m) with q(m) being the wave number of the first-order scattering maximum was found to obey the following scaling law: D similar to (phi (p)/T)N-1/3(n)2/3 for phi (p)/T greater than or equal to (phi (p)/T)(MF) similar to N-n(1/2), and D similar to (phi (p)/T)N-0(n)1/2 for phi (p)/T less than or equal to (phi (p)/T)(MF) for the copolymer systems covered in this work. Here (phi (p)/T)(MF) is a crossover value of phi (p)/T from the mean-field disordered state to the non-mean-field disordered state. These results are consistent with our previous experimental observations [Hashimoto et al., Macromolecules 16 (1983) 1093]. The results obtained here together with our previous results of chi (eff) similar to (phi (p)/T)N-n(-1/2) and D-0 similar to N-n(1/2) (Mori et al., J Chem Phys, 104 (1996) 7765) give the following scaling law: D/D-0 similar to (chi N-eff(n))(1/3) for (chi N-eff(n)) greater than or equal to (chi N-eff(n))(MF) and D/D-0 similar to (chi N-eff(n))(0) for (chi N-eff(n)) less than or equal to (chi N-eff(n))(MF) where chi (eff) is an effective segmental interaction parameter between polystyrene and polyisoprene segments at a given phi (p) and T for a given copolymer, and Du is the wavelength of the dominant mode of the concentration fluctuations of the copolymer systems in the mean-field disordered state. (chi N-eff(n))(MF) is the crossover value of chi N-eff(n) at the mean-field disordered state to the non-mean-field disordered state, which depends on f(PS). (C) 2001 Elsevier Science Ltd. All rights reserved.