This book contains the ECNI mass spectra of 361 compounds selected to include the major classes of environmental con- taminants, such as those found on the U.S. Environmental Protection Agency's priority pollutant list. Compound clas- ses of environmental importance include halogenated benzenes and phenols, nitrobenzenes and the related dinitro herbicides, polycyclic aromatic hydrocarbons, DDT derivatives, hexachloro- cyclopentadiene pesticides, halogenated biphenyls, dioxins, and dibenzofurans. The spectra, measured with the ion source at 100 oC and 250 oC, are reported graphically. In addition, compound name, Chemical Abstracts registry number, molecular weight, and structure are given. This collection of reference spectra will be of use to environmental scientists and to analytical chemists concerned with the sensitive detection and identification of synthetic chemicals in a wide variety of matrices.