The thermal stability of selenite, selenate, selenomethionine and trimethylselenonium was studied using different chemical modifiers in various amounts. The normally recommended amounts of nickel nitrate, magnesium nitrate, copper nitrate, copper nitrate mixed with magnesium nitrate, palladium nitrate and palladium nitrate in combination with magnesium nitrate were investigated. None of the studied modifiers stabilized the four species to the same extent. In aqueous solutions, addition of 2.7 mug of Pd or 7.5 mug of Pd + 5 mug of Mg(NO3)2 stabilized selenite, selenate and selenomethionine equally, while the sensitivity of trimethylselenonium was only 55% using palladium and 85% using palladium and magnesium nitrate. These chemical modifiers were used for the determination of selenium in serum. In aqueous solution, addition of 11 mug of Pd + 1128 mug of Mg(NO3)2 resulted in an equal Stabilization of selenite, selenate and trimethylselenonium, while the sensitivity of selenomethionine was 15% higher. This chemical modifier was not applicable to serum and urine, as a white layer accumulated in the graphite tube during the experiments. Using the modifier containing 7.5 mug of Pd + 5 mug of Mg(NO3)2 for analysis of National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) Freeze-dried Urine the selenium concentration was 27.2 +/- 1.3 mug l-1 (certified value, 30 +/- 8 mug l-1). The result of the analysis of the serum reference material, Seronorm, was 96.6 +/- 1.8 mug l-1 of Se (recommended value 96 mug l-1). This chemical modifier is proposed as the best choice among the examined modifiers for determination of selenium in serum and urine