Ten selected metals, namely Zn, Cr, Cd, Cu, Ni, Pb, Fe, Mn, Mg and Ca, commonly present in surface waters were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). For all quantitative analyses, basic metrological characteristics were obtained, including estimation of combined and expanded uncertainties. Special attention was given to ascertaining which component of uncertainty plays a dominant role in the overall determination process (from sampling to final results), and particularly, in ICPOES calibration and measurement. The largest contribution to the combined uncertainty of the final analytical determination of the element concentrations was due to sampling and calibration, as well as general imprecision of the whole analytical procedure.