We have conducted low energy X-ray spectroscopy of ion/surface interactions with superconducting tunnel junction (STJ) detectors. The STJ detectors (Nb-Al-AlOx-Al-Nb) developed at the Lawrence Livermore National Laboratory have a high energy resolution and a high count-rate capability. The detectors are operated in an adiabatic demagnetization refrigerator with 60 mK base temperature at the end of a 40 cm long cold finger, which can be inserted into a UHV sample chamber connected to an ion source. We have studied the interaction of Ar{sup 9+}, O{sup 7+}, N{sup 6+} and C{sup 5+} with various targets (Au, SiH and SiO{sub 2}) at a kinetic energy of 10 keV/q. The resolution of 12-15 eV below 1 keV allows the different satellite lines emitted by these ions to be separated.