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This report presents data on the chemistry of precipitation and storm runoff that were collected during 29 storms from July 1979 through November 1980, in an urban environment in the Pennypack Creek basin, Philadelphia, Pennsylvania. Daily mean and instantaneous stream discharge data were collected at two U.S. Geological Survey gaging stations. Water-quality data collected from these sites and from one storm-sewer sampling site included nutrients, chemical and biochemical oxygen demands, solids, metals, major anions, other constituents, and pH. Instantaneous loads of selected constituents were computed. Chemical oxygen demand, biochemical oxygen demand, sulfate, alkalinity, chloride, and dissolved solids generally were low in precipitation and runoff. During the November 5, 1980 storm at Pine Road, dissolved nitrate concentrations equaled or exceeded 10 milligrams per liter in 17 percent of the samples analyzed and dissolved ammonia nitrogen concentrations exceeded 2.5 milligrams per liter. Generally, a comparison of median concentration of copper, lead, and zinc in precipitation and runoff at the Tustin Street stormsewer site showed that median concentrations in precipitation were greater than those in runoff. DATA-COLLECTION SITES Precipitation Precipitation collectors were installed at three sites (fig. 1) in the Pennypack Creek basin in Philadelphia. One precipitation sample per storm was collected in open plastic buckets located on the roofs of two buildings and in a tree. Precipitation was not sampled for all storms. Personnel arrived onsite as soon as possible after the storm to collect and deliver samples to the two City of Philadelphia laboratories for chemical analysis. Runoff Gage height was recorded continuously at the two sites on Pennypack Creek (table 1) and converted to discharge. Daily mean discharge for sampled storms is given in table 2.