The development of high-resolution gas source mass spectrometry has permitted entirely new types of measurements of multiply-substituted isotopologues in gas species of geochemical significance. Here, we present recent advances afforded by measurements of 15 N 15 N in natural samples, together with 14 N 14 N and 15 N 14 N. We show that the abundance of the doubly-substituted 15 N 15 N isotopologue in hydrothermal gases, often mixtures of volatiles of widely different origins, allows tracing the provenance of nitrogen. The approach is based on the recent finding that atmospheric N2 has a substantial enrichment in 15 N 15 N of nearly 20‰ relative to any other source of N2. This is particularly useful for the study of hydrothermal gases, where characterizing the isotopic composition and provenance of volcanic N2 is important for a wide range of applications in high-temperature geochemistry, but where air-derived N2 is unavoidable. In this review, we summarize the evidence that 15 N 15 N is an unambiguous tracer of air