Faecal indices of dietary quality can provide useful knowledge about the general ecology of a species, but only if the measurements are accurate and the results are interpreted with caution. In this article, we evaluated the potential of near-infrared spectroscopy (NIRS) as an analytic tool to derive faecal indices of dietary quality in an omnivorous monogastric species with a wide dietary range, i.e. the brown bear Ursus arctos. We also tested the effects of field exposure on faecal constituents (i.e. nitrogen, lignin, crude fiber (CF), ether extracts (EE), acid detergent fiber (ADF), neutral detergent fiber (NDF), ash and dry matter (DM)), which are commonly used as faecal indices of dietary quality. We collected 172 faecal samples from 45 GPS-marked brown bears in south-central Sweden between May and October 2010. For each sample, we recorded maximum field exposure time (in hours) and canopy cover (in %). We used multivariate partial least-squares regression with a segmented cross validation procedure to calibrate the NIRS method. We obtained very good (r² ≥ 0.9) NIRS validation results for faecal nitrogen content and NDF, and good (0.7 ≤ r² < 0.9) results for lignin, CF, EE, ADF and ash. Validation results for DM were poor (r² = 0.29). We found that field exposure time negatively affected faecal nitrogen content, especially during the first 40 hours of exposure. Because CF and NDF are strongly negatively correlated with faecal nitrogen content, concentrations of these two components increase as a consequence of field exposure. Faecal EE content appeared to be stable under field conditions. Our conclusions are twofold. First, NIRS can be an accurate, fast and inexpensive analytical tool to evaluate certain faecal indices of dietary quality, including for omnivorous species. Second, faecal indices of dietary quality can be affected by field exposure and can vary among individual animals. Ignoring individual variance and the effects of field exposure on faecal indices of dietary quality may cause bias in research findings.