[en] High-resolution (s/sub M/<0.25 amu) observations have been made of the isotopic composition of oxygen, neon, and magnesium in the galactic cosmic rays (approx.100-300 MeV/amu). Derived source abundances shown enhancements of ²²Ne/²⁰Ne, ²⁵Mg/²⁴Mg, and ²⁶Mg/²⁴Mg over solar-system values by factors of 4.1/sup +0.8//sub -0.6/, 1.6/sup +0.3//sub -0.2/, and 1.6/sup +0.3//sub -0.2/, respectively. These data demonstrate that the cosmic-ray source is compositionally distinct from the solar system and constrain nucleosynthesis models, several of which are considered