The determination of Cr in the presence of various isomorphous metals has been studied. The atomic absorption signal for Cr was increased and stabilized by the presence of 20 µg of Mg(NO3)2, 1 µg Rh and 1 µg Pt. Magnesium, Rh and Pt gave comparable characteristic masses of 3.2, 3.0 and 2.8 pg, respectively, when integrated absorbance was measured. The limits of detection were 0.18, 0.14 and 0.091 µg 1–1, respectively. The efficiency of these modifiers was tested with the direct determination of Cr in rain-water and serum samples. Quantification was performed with aqueous standards in the case of the rain-water samples, and with matrix-matched standards in the case of the serum samples. Recovery tests and a serum reference material were used to check the accuracy of the proposed methods. Accurate results and good agreement with the certified serum values were found in the presence of platinum as a modifier. Chemical modifiers were not necessary for the determination of Cr in rain water.