An analysis has been made of a sample of rain water collected in Washington, D.C. following a sustained photo-oxidant smog alert which was accompanied by haze and low visibility. The analysis was effected by purging the organic content from the water with helium gas and identifying the constituents by means of gas chromatography—mass spectrometry. The predominant compound was found to be 3-methylfuran, a possible intermediate or end product of the atmospheric decomposition of terpenoid hydrocarbons. The analysis suggests that the smog resulted from the air oxidation of hydrocarbons evolved from Appalachian forestation rather than from manassociated activities.