Two main objectives of the study were to compare the distribution of hydrocarbons in the atmosphere with those observed in sources and to estimate the degree to which hydrocarbons of different reactivity participate in the photoreaction. Typical photochemical smog hydrocarbons can be described as auto exhaust plus gasoline vapors plus natural gas. Samples taken near specific sources (petrochemical plants, brush fires, oil fields) show distinctly different distributions. Afternoon smog samples show substantial attenuation of the more reactive hydrocarbons (specifically the olefins). By study the relative amounts of acetylene, ethene and propene it was possible to estimate the extent and time of reaction. Irridation produces continued reaction for 24 hours or more. Sampling and analysis procedures are described.