The initial stages of nucleation during liquid-liquid phase separation in mixtures of high molecular weight polymers was studied by time-resolved small angle neutron scattering. Phase separation was induced either by decreasing temperature or by increasing pressure. One of the blend components was labeled with deuterium to obtain sufficient scattering contrast between the components. The general features of nucleation were independent of quench depth and the nature of the quench (temperature quench versus pressure quench). The early stages of nucleation consisted of amplification of concentration fluctuations. During this stage, the scattered intensity (I) in the low scattering vector (q) limit was consistent with the Ornstein-Zernike equation. This enabled the determination of the characteristic length scale of the growing fluctuations, xi. The I vs q behavior at intermediate scattering vectors (q > 1/xi) could be described by a power law (I similar to q(-d)). We demonstrate the existence of a time-temperature superposition principle during nucleation: The time dependence of d at different quench depths could be superimposed by a lateral shift of the data along the time axis (log scale). In analogy to the shift factor for viscoelastic behavior of polymers, we define a nucleation shift factor, a(N), which describes the slowing down of nucleation kinetics with decreasing quench depth. Similarly, nucleation after pressure quenches can be described by a time-pressure superposition law. For each quench, we find that the scattering intensity is independent of time in the high q regime (q > q(merge)). This implies the absence of growing structures with length scales smaller than xi(crit)=1/q(merge) during nucleation. This aspect of nucleation is consistent with classical theories which predict the existence of a critical nucleus size. As expected, xi(crit) increases with decreasing quench depth.