Time-resolved light scattering experiments have been performed on the two-step phase separation process for a binary mixture of deuterated polybutadiene and protonated polyisoprene with the critical composition. The first-step phase separation was induced by quenching the system from a temperature T-0 in a single-phase state to T-1. At various times in the late stage spinodal decomposition (SD) at T-1, the mixture was further subjected to the second-step phase separation by temperature jump (T-jump) from T-1 to T-2 in such a way that the driving force for the phase separation increases and hence phase separation process is accelerated. The comparison between the single-step phase separation behavior from T-0 to T-1 or to T-2 and the two-step phase separation behavior was made in terms of the time evolution of the main scattering peak arising from a dominant mode of the composition fluctuations developed in the phase separation process. To do so, we proposed and applied a new scaling method for the scattering peak position, q(m,1), and intensity, I-m,I-1, to explore the time evolution of the fluctuations for the two-step phase separation process. The new scaling method, which takes into account an abrupt change in the spatio-temporal scale of phase-separating system involved by the second-step T-jump clearly elucidates a nonlinear pathway according to which the initial structures developed in the first-step process is relaxed and transformed toward an equilibrium structure at T-2 after the second-step process. (C) 2000 American Institute of Physics. [S0021- 9606(00)51115-5].