The influence of the zeta potential on the sorption between microorganisms (Saccharomyces cerevisiae and Escherichia coli) and iron oxide nanoparticles is demonstrated in a model salt solution at two different pH-values. There was only a 1% survival rate of E. coli (4.5 x 10(7)cells/mL) in the presence of 24 mg/L nanoparticulate iron oxide at pH 4. S. cerevisiae were less affected by the presence of the nanoparticulate iron oxide. The extent of iron oxide nanoparticle coverage on the surface of the microorganisms appears to be related to electrostatic interaction forces. Furthermore, the toxic effect of the nanoparticle concentration follows the sorption isotherm for E. coli. Based on the resulting hydrodynamic size distributions in the supernatant after sorption experiments, it could be shown that predominantly smaller particle aggregates oxide were sorbed onto E. coli. This was evident by a shift in the particle size distribution towards a larger mean particle size. The effect was observed to a lower extent for S. cerevisiae. The extent of iron oxide nanoparticle sorption on E. coli quickly reached a maximum and remained constant during a 24 h period compared to S. cerevisiae where sorption increased over time.