This paper describes the operation and application of a portable cavity ring-down spectrometer (CRDS) designed to measure the isotopic composition of carbon dioxide. The instrument is capable of measuring δ13C for CO2 concentrations ranging from atmospheric (400 ppm) to 100%, at precisions and accuracies that are comparable to laboratory-based gas source mass spectrometers. This flexibility and portability are ideal for applications on active volcanoes, and it is now possible to obtain isotopic measurements on a near real-time basis. We show applications of the CRDS for soil gases on volcanoes and in calderas, for characterizing the isotopic composition of a volcanic plume, and for measuring the temporal variability of δ13C in the atmosphere. Future directions hold the potential to use volcanic gas isotopes for monitoring purposes, and to combine different isotopic systems to reveal the source or sources of gas.