The authors report the application of the internal standard method to the simultaneous determination of trace elements in environmental reference materials. The standard soil material used was IAEA CRM Soil-5. The power plant fly ash reference used was NBS SRM-1633a. Fifteen target elements, including As, Ba and Ce, were determined. Internal standards were supplied by six elements, including Na and Mg. Although there were several interfering elements, their effect could be eliminated by utilizing more than one gamma-ray peak and carrying out appropriate corrections. The values determined for most of the target elements were well within the certified range. Measured concentrations were of the orders of 10 to 1000 ..mu..g/g. 6 references, 2 figures, 5 tables.