Several off-the-shelf passive tagging techniques were used to evaluate the viability of Maine's Oil Conveyance Law to determine the origin of mystery oil spills. Duplicating the operating conditions of the law, ship's samples of all crude oil shipments received during a 15-day period at Portland Harbor were collected. Selected crude oils were subjected to weathering in 500-gallon tanks exposed to the outdoor ambient conditions of Portland, Maine. Subsamples of the weathering spill were taken at several intervals up to 15 days. Attempts were made to correctly match the weathered “unknown” to one of the collection of ships’ samples by passive tagging. Techniques employed included the comparison of gas chromatograms by several procedures, vanadium-nickel ratios, sulfur-nitrogen ratios, and infrared spectra. In a preliminary review of the data, successes and limitations are discussed.