Summary Syneresis is commonly believed to be incompatible with the application of polymer gels to reduce the permeability of porous media. In this paper, it is shown that, although the permeability of a geltreated porous medium does increase as syneresis proceeds, the degree of permeability reduction in cores remains technologically useful even when 95% syneresis is observed in bulk samples. Arguments are presented for the preferential shrinking of syneresed gel into pore throats; this model explains why permeability reduction is maintained and accounts for several other experimental results. The absence of performance penalties for syneresis has significant implications for the applicability of gels for water shutoff treatments in matrix formations. For example, it raises the possibility that polymer gels can be applied successfully in situations previously considered unfeasible because of the difficulty of maintaining gel stability, e.g., at high temperatures or in the presence of hard formation brines.