The spinodal decomposition (SD) of a critical mixture of deuterated and protonated polybutadiene of nearly equal chain lengths was investigated. This mixture has an upper critical solution temperature type phase diagram and the spinodal temperature at the critical point is 99.2 °C. Phase separation was induced by quenching a single-phase specimen at an initial temperature, T i (=102.3, 123.9, and 171.6 °C), to a final temperature, T f (=-7.5, 1.1, and 10.5 °C). The subsequent SD was followed by time-resolved small-angle neutron scattering. The Onsager coefficient, Λ(q;Tf), as a function of wave number q and Tf, derived from experimental growth rates, R(q;Tf), of the Fourier mode of concentration fluctuations and estimation of ST(q;Tf), was compared to the reptation model theories of Pincus and Binder. Experimental Λ(q;Tf) was found to give a q-dependence greater than that given by the theories. Here, ST(q;Tf) denotes the virtual structure factor at Tf inside the spinodal region. The reduced wave number Qm(τ) and intensity S̃m(τ) at the peak of the scattering structure factor in the early and intermediate stages of SD were found to be scalable in terms of a reduced time τ when Ti was fixed and Tf was varied, but not when Tf was fixed and Ti was varied. The failure of the scaling law in the latter instance may be attributed to the fact that the concentration fluctuation at the onset of SD has a different memory of the thermal concentration fluctuation in the single-phase region depending on Ti, which affects the subsequent SD over an extended period of time.