Previous studies on polylactide (PLA)/poly(epsilon-caprolactone) (PCL) biodegradable blends revealed enhanced nucleation of PLA. The mechanism by which this enhancement of nuclei density occurs attracted significant interest. In this study, precursors' transportation from PCL into PLA phase is invoked in order to interpret the experimental findings related to phase separation and crystallization. This mechanism is supported by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The DSC data revealed that PCL crystallinity within the blends is decreased with PLA content. In addition, POM data showed more nuclei within PLA as the PCL weight fraction is increased. The linear viscoelastic properties of these blends in the vicinity of the phase separation and cold crystallization boundary are examined. It is shown that the combination of POM and rheometry allows determining the phase diagram of the blend and identifying changes occurring due to phase separation and crystallization. Thus, the complete phase diagram of such systems can be fully determined, essential in process design and optimization.