A storm water catch basin hazardous liquid valve that is automatically responsive to the presence of hazardous liquids in storm water run-off, the valve utilizing a hydrophobic barrier member to separate hazardous liquids from water. Collection of a sufficient amount of hazardous liquids across the hydrophobic barrier member initiates a float release member to seal off the catch basin. In the neutral or non-activated status, the storm water run-off enters the catch basin, passes through the valve assembly and out through the storm water conduit system in normal manner. When a hazardous liquid, such as oil, gasoline, chemicals, etc., is present in the storm water run-off above a threshold amount, the valve activates to close off the catch basin such that the storm water run-off is precluded from entering the storm water conduit system.