Many low-permeability reservoirs contain large resources. Imbibition is fundamental to oil recovery from such reservoirs under most secondary and improved recovery processes, and is characteristic of porous-medium wettability. The rate and extent of imbibition depend on the viscosities of the wetting and nonwetting phases. A new formulation for the dimensionless time appropriate to countercurrent imbibition was developed, and a new scaling was used to correlate experimental and simulation results that improves correlation significantly by taking endpoint fluid-phase mobilities and the mobility ratio into account.