This method is for determining rhenium in fresh and used platinum-containing catalysts in the range of 0.05 to 0.75%. Elements that produce colored ions in the dissolution or color-development step constitute a positive interference. Iron or copper in moderate concentrations are reduced by stannous ion to a colorless state and, therefore, do not interfere. Molybdenum which forms a red coloration with the stannous ion is the major interference. Directions for the separation of rhenium from molybdenum are given. Analysis for molybdenum is by UOP Method 833.