The characteristics of sputter-initiated resonance ionization spectroscopy (SIRIS) for determining trace uranium concentrations in soils are investigated. Although, presently, the most formidable problem in the assay of environmental materials with SIRIS is the low sensitivity of the system, we have chosen to focus on matrix effects as possible limiting factors in this paper. Separate active efforts are being made to improve sensitivity and isotopic selectivity in RIS systems. Electrically conducting solid samples compatible with high vacuum are made by compacting soil with 15% or greater graphite binder. It is found that matrix effects on the absolute uranium SIRIS signals can be as large as two orders of magnitude, precluding a direct comparison of uranium concentrations through uranium SIRIS signal levels. The method of isotope dilution is also explored. Systematic errors caused by different molecular forms or different microscopic physical locations of the two isotopes are less than 30% in this method. It is found that for samples in which these effects are minimized, the capability of the instrument is such that uranium concentrations can be determined successfully to better than 10% accuracy.