Some commonly used chemical modifiers such as magnesium, palladium and calcium nitrates, ammonium phosphate and some of their mixtures, were tested in this work for the determination of main contaminant elements, considering cadmium, chromium, iron, lead and selenium, in natural waters by electrothermal atomic absorption spectrometry. For each element, various modifiers were used, temperature programs were experimentally selected and analytical performances (calibration equation, linear regression coefficient, detection limit, characteristic mass and precision at low and high level of analyte) were evaluated. The results obtained indicate that magnesium nitrate (5 μg Mg) can be proposed as chemical modifier for these five elements in natural waters. The results of recovery experiments were between 96 and 109% for five elements under study. The procedure was applied in the analysis of underground waters and satisfactory agreement was achieved between external calibration and standard addition results.