The accurate determination of ignition loss in pure rhodium powder and iridium powder products is an important index for the metal balance and fair trade of production materials. The ignition loss (the content of volatile substances) is usually determined by thermogravimetry. But this method has some disadvantages such as high cost and long period of analysis. Based on the principle that the volatile substances were easily removed by hydrogen reduction at high temperature, the determination of ignition loss in pure rhodium powder and iridium powder by hydrogen reduction gravimetry was proposed. The following experimental conditions were obtained: the sample amount was 1.0-2.0g; the hydrogen reduction was conducted by temperature-gradient method, i.e., the temperature was increased from room temperature to 200℃ and kept for 30min, and then increased to 400℃ and kept for 30min, and then increased to 600℃ and kept for 30min, and finally increased to 800℃ and kept for 120min; the quartz boat containing hydrogen-reduced rhodium powder and iridium powder was cooled in desiccator for 30min followed by constant weight. The further experiments indicated that the influence of cooling on analysis results could be ignored in presence of hydrogen or nitrogen. The possible carbon content in sample had no interference with the determination. The volatile substances in rhodium powder and iridium powder samples were mainly composed of oxygen and nitrogen. The proposed method was applicable for the determination of ignition loss of pure rhodium powder and iridium powder products with total mass fraction of nonvolatile impurity metal elements less than 0.010%. The ignition loss of control sample and actual sample (0.0042%-0.45%) of rhodium powder and iridium powder was determined according to the experimental method. The found results were basically consistent with the reference values or those obtained by thermogravimetry. The relative standard deviations (RSD, n=11) were less than 5.6%.