The study explores the possibility of determining whether airborne hydrocarbons are predominantly natural or man-made by measuring their carbon isotope composition. Difficulties were encountered in finding a collection system that would quantitatively collect hydrocarbons while effectively discriminating against the large volumes of water vapor and carbon dioxide which dominated the effluent stream. The various unsuccessful approaches to making collections from living plants are described in the report. The final collection method consisted of warming pine needles to 40C and passing nitrogen through them in a closed container. Hydrocarbons were collected in a dry ice cooled trap, purified, combusted to carbon dioxide and analyzed for carbon isotope composition. The composition of pure terpene extracts was also determined to provide a basis for interpreting the data from the effluent volatiles. The study concludes that some plants produce hydrocarbons which may be distinguished from petroleum hydrocarbons while others do not.