The recent progress with ion traps coupled to soft x-ray beamlines makes it possible to study K-edge x-ray absorption spectroscopy (NEXAFS) of small molecular cations in gas phase. This opens up for new fundamental insights to molecular physics since additional electronic states can be reached compared to the neutral molecule. Here we propose to study high resolution NEXAFS of molecular N_2^+, O_2^+ and CO^+ in the ground state using an ion trap. In addition to adding a new dimension to fundamental molecular physics, these measurements are also of interest for astro-, atmosphere-, and plasmaphysics.