We present experimental studies of the mixing process of a homopolymer blend of poly(methylphenylsiloxane) (PMPS) with polystyrene (PS). The system is first allowed to decompose spinodally at low temperature for few minutes and next is heated to a higher temperature to the one-phase or two-phase (metastable or unstable) region. In all cases the intensity drops initially after the temperature jump. In the one-phase region the intensity drops to the base scattering. In the metastable region it drops initially, and later on it starts to grow. In this region the peak position shifts strongly toward small wavevectors, and the intensity drops considerably. Finally, in the spinodal (unstable) region the peak position shifts toward smaller wavevectors, but the intensity of the peak hardly changes before increasing again. The decrease of the peak intensity is exponential with the characteristic decay time which approaches infinity when the temperature of the jump approaches that of the quench. The mixing process mainly involves the interdiffusion of polymers, without global movement of the interface. Small domains disappear faster than larger domains, and, therefore the peak position (indicating an average size of the domains) shifts toward smaller wavevectors. In the metastable region the average wavevector as a function of time has a characteristic minimum, which shifts toward zero as we approach the spinodal. If the jump is made to the unstable region, the average wavevector monotonically decreases with time. The average wavevector, after temperature jump, allows to predict the location of the binodal and spinodal. We call this new method of spinodal location a jump spinodal specification method (JSS method). The generic features of this method have been confirmed in the computer simulations of the Flory-Huggins-de Gennes model with the Langevin dynamics for the mixture of polybutadiene and deuterated polybutadiene.