An effective and commonly used technique for studying the sources, photochemistry, and even the ''photochemical age'' of trace species is to examine ratios of hydrocarbons by assuming the ratio is independent of transport processes. We present results from mesoscale model calculations that suggest a significant effect by atmospheric mixing on the ratio. We also show that the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport. Without additional information, it is not possible to derive a unique value for the age of an air mass from hydrocarbon ratios alone. ¿ American Geophysical Union 1993