Recently, the addition of poly(ethylene glycol) (PEG) homopolymers was found to significantly lower the sol-gel transition temperature T-gel of the aqueous system of a block copolymer composed of poly(lactic acid-co-glycolic acid) (PLGA) and PEG, different from the "normal" phenomenon of a T-gel increase for Pluronic hydrogels of block copolymers of PEG and poly(propylene glycol) (PPG). While such an effect is useful for applications of PLGA-PEG-PLGA physical hydrogels as an injectable biomaterial, it is puzzling why the addition of a hydrophilic homopolymer probably enhances the hydrophobic-association-induced physical gelation of ail amphiphilic block copolymer. A series of PEG homopolymers with different molecular weights and concentrations were examined in the aqueous systems of PEG/PLGA block copolymers with three compositions in the present paper. An immiscibility mechanism was put forward by us to account for the earlier spontaneous thermogelling under the assumption of the structure of a randomly percolated micelle network. Meanwhile, the late physical gelling of Pluronics in the presence of PEG homopolymers was interpreted by an order-interfering mechanism. We further found that the immiscibility mechanism might also work for Pluronics upon addition of ethylene glycol and oligo(ethylene glycol).