This method is for determining rhenium in relatively pure perrhenic acid and ammonium perrhenate over a concentration range from 40 to 70 mass-%. Molybdenum is an interference when it is present at concentrations higher than 10% of the rhenium content. Molybdenum can be determined by UOP Method 833. Other elements that produce colored ions in the dissolution or color-development step constitute a positive interference.