The purpose of the presented work was to investigate the error in the experimental determination of metal uptake during the batch equilibrium biosorption process. The objective was to quantify the inaccuracies in mass and volume measurement as well as calibration, drift and fluctuation of the Atomic Absorption Spectrophotometer readings for an example of Cd, Cu or Zn sorption by protonated Surgarsum fluituns biomass. As a result, a mathematical description of the total error as a function of the initial and final metal concentrations, dilution factor, calibration wavelength, solution volume, sorbent mass and sorbed species was developed. Dilution and insufficient difference between the initial and fmal concentrations of the sorbed metal were determined to be the largest error sources. A recommendation for the choice of experimental conditions such as the initial concentration and the corresponding solid to liquid ratio is made, so that excessive experimental errors can be avoided. The discrepancies between experimental data and predictions of a previously developed model for metal uptake in multi-metal-systems were found to be of a similar magnitude as the estimated experimental error. Therefore, no significant modeling error could be detected. INTRODUCTJON