The molecular ion H-3(+) is considered the cornerstone of interstellar chemistry because it initiates the reactions responsible for the production of many larger molecules, Recently discovered in dense molecular clouds. H-3(+) has now been observed in the diffuse interstellar medium toward Cygnus OB2 No. 12, Analysis of H-3(+) chemistry suggests that the high H-3(+) column density (3.8 x 10(14) per square centimeter) is due not to a high H-3(+) concentration but to a long absorption path. This and other work demonstrate the ubiquity of H-3(+) and its potential as a probe of the physical and chemical conditions in the interstellar medium.