The isotopic compositions of galactic cosmic ray boron, carbon, and nitrogen have been measured at energies near 300 MeV amu-1, using a balloon-borne instrument at an atmospheric depth of ~ 5 g cm-2.The calibrations of the detectors comprising the instrument are described. The saturation properties of the cesium iodide scintilla tors used for measurement of particle energy are studied in the context of analyzing the data for mass. The achieved rms mass resolution varies from ~ 0.3 amu at boron to ~ 0.5 amu at nitrogen, consistent with a theoretical analysis of the contributing factors.Corrected for detector interactions and the effects of the residual atmosphere, the results are 10B/B = 0.33+0.17-0.11, 13C/C = 0.06+0.13-0.01, and 15N/N = 0.42+0.19-0.17. A model ofgalactic propagation and solar modulation is described.Assuming a cosmic ray source composition of solar-like isotopic abundances, the model predicts abundances near earth consistent with the measurements.
