A total of 3024 compounds were procured to measure high quality mass spectra for inclusion in the NIH/EPA mass spectral data base. Compounds were assayed for purity before mass spectra were measured and, when necessary, were purified by thin-layer chromatography, recrystallization, sublimation, or distillation. Compounds that were sufficiently volatile to be amenable to gas chromatography (GC) and were less than 99% pure were introduced by GC. Quality control procedures included the use of decafluorotriphenylphosphine (DFTPP) for the molecular leak and GC inlets and cholesterol for the direct insertion probe inlet. Spectra of these standards were required to meet stringent acceptance criteria every four hours of instrument operation. High quality mass spectra were measured for 2000 of the 3024 compounds procured.