The effect of various states of weathering (no weathering, 20% evaporatively weathered, and emulsification) on the effectiveness of oil dispersants Corexit 9527 and 9500 in dispersing Alaska North Slope crude oil into the water column was tested under laboratory conditions at a combination of realistic subarctic salinities and temperatures. A modified version of the swirling flask effectiveness test was conducted at temperatures of 3, 10 and 22 degreesC with salinities of 22parts per thousand and 32parts per thousand. Petroleum dispersed into the water column following application of dispersant was measured by gas chromatography with flame ionization detection. Based on comparison of unresolved complex mixtures, dispersants dispersed less than 40% of the fresh oil and less than 10% of the weathered oil and were most effective (25-75%) when used to disperse a stable oil/water emulsion at 10 degreesC. At the combinations of temperature and salinity most common in the estuaries and marine waters of Alaska, dispersants effectiveness was less than 10%, the detection limits of the tests. The results indicate that oil weathering state, seawater salinity and temperature are important factors affecting dispersant performance, however because our laboratory tests were conducted at low mixing energy, considerable caution should be used in extrapolating these laboratory studies to field conditions. Published by Elsevier Science Ltd.