A study of the analysis of benzene, toluene, ethylbenzene and xylenes by static headspace gas chromatography illustrated the estimation of uncertainty associated with the influence of several parameters, such as preparation of the standard solutions, calibration of the instrument, recovery and limit of detection. The objective of the investigation was to indicate the contribution of each stage in the developed procedure on the uncertainty measurement of the determination of volatile aromatic hydrocarbons in surface and underground water. The uncertainty sources were identified and illustrated in an effect diagram. The data illustrated the evaluation of uncertainty by the quantification of the uncertainty arising separately from each source. It has been demonstrated that data can be used to estimate the uncertainty resulting from the combined effect of all sources.