The phase separation process in a critical mixture of polydimethylsiloxane and polyethylmethylsiloxane (PDMS/PEMS, a system with an upper critical solution temperature) was investigated by time-resolved light scattering during continuous quenches from the one-phase into the two-phase region. Continuous quenches were realized by cooling ramps with different cooling rates kappa. Phase separation kinetics is studied by means of the temporal evolution of the scattering vector q(m) and the intensity I-m at the scattering peak. The curves q(m)(t) for different cooling rates can be shifted onto a single mastercurve. The curves I-m(t) show similar behavior. As shift factors, a characteristic length L-c and a characteristic time t(c) are introduced. Both characteristic quantities depend on the cooling rate through power laws: L(c)similar tokappa(-delta) and t(c)similar tokappa(-rho). Scaling behavior in isothermal critical demixing is well known. There the temporal evolutions of q(m) and I-m for different quench depths DeltaT can be scaled with the correlation length xi and the interdiffusion coefficient D, both depending on DeltaT through critical power laws. We show in this paper that the cooling rate scaling in nonisothermal demixing is a consequence of the quench depth scaling in the isothermal case. The exponents delta and rho are related to the critical exponents nu and nu* of xi and D, respectively. The structure growth during nonisothermal demixing can be described with a semiempirical model based on the hydrodynamic coarsening mechanism well known in the isothermal case. In very late stages of nonisothermal phase separation a secondary scattering maximum appears. This is due to secondary demixing. We explain the onset of secondary demixing by a competition between interdiffusion and coarsening. (C) 2004 American Institute of Physics.