A study of the behavior of (CrO3)-Cr-51 in 70% HClO4 over the temperature range from 20 to 194 degrees C by means of Cr-51 labelling, W-VIS spectrophotometry and ion exchange chromatography, shows that the solubility of (CrO3)-Cr-51 depends on a competition between the dissolution process and the acid reduction of solution phase Cr(VI). These processes occur simultaneously and are dependent on both the temperature and the concentration of Cr(VI), as shown by comparison between radiometric measurements (where total chromium can be accurately determined) and spectrophotometric measurements (where only the Cr(VI) is detectable at the wavelengths studied). These conclusions are confirmed by PbCrO4 precipitation of Cr-51(VI), where at 194 degrees C, 97% of the total chromium appears as (PbCrO4)-Cr-51 while at 86 degrees C only 5% does. Cation exchange chromatography of the solution after brief contact of (CrO3)-Cr-51 with concentrated HClO4 at 20 degrees C shows only traces of Cr-51(VI), most of the radioactivity eluting as Cr-51(H2O)(6)(3+), with smaller amounts of species with +2 and +1 charges. These results imply serious limitations to the spec trophotometric determination of low concentrations of total chromium in alloys or in biological material which use dissolution in 70% HClO4 as a primary analytical step.77416719197