The thermal stabilities of selenite, selenate, selenomethionine and trimethylselenonium in aqueous solution and plasma were compared, The stabilities of the four selenium forms were different in aqueous solution and plasma, The nitric acid concentration had little influence on the sensitivity of the species, The presence of chloride affected the stability of the selenium forms differently in aqueous solution, while there was no pronounced effect on the stabilization in plasma, Different amounts of palladium, varying from the application of 2 to 40 mu g into the graphite tube, were compared, The application of 20 mu g of palladium showed the best result in terms of sensitivity and equal stabilization of the selenium species when analysing plasma, Different amounts of magnesium nitrate, varying from the application of 0.6 to 24.3 mu g of magnesium into the graphite tube, were examined, The addition of magnesium nitrate did not improve the results, either in terms of sensitivity or equal stabilization, but only increased the background signal, On adding 20 mu g of palladium, selenite and selenate were equally stabilized, while the sensitivity of selenomethionine was about 80% of that of the former species, It was not possible to stabilize trimethylselenonium to the same extent with this modifier, Peak shapes and appearance times of the atomization signals were equal for the four selenium species with this modifier, The addition of 20 mu g of palladium was used for the analysis of the serum reference material Seronorm Trace Elements Serum, resulting in a mean of 85.4 mu g l(-1) (n = 16) and a standard deviation of 6.6 mu g l(-1), corresponding to a precision of 3.9% (recommended value 86 mu g l(-1)), This modifier is proposed as the best choice for the determination of selenium in plasma