Particle deposition rates in quartz beds were compared with model predictions near the isoelectric point of silica (pHiep≈2–3). Deposition rates, expressed as attachment efficiencies, were determined for 248-nm sulfate latex and 245-nm carboxyl latex within the pH range of 2–5, at NaCl-adjusted ionic strengths of 10−1.5 and 10−2 M. Using other 1:1 electrolytes, the effect of cation type was evaluated. Theoretical deposition rates based on DLVO theory were determined using constant charge and constant potential electrical double layer (EDL) interaction expressions. The experimental attachment efficiencies, αexp, for the sulfate latex were substantially lower than either of the theoretically predicted attachment efficiencies, evidence that an additional repulsive interaction force is important near the pHiep of quartz. This force was absent from the carboxyl latex deposition experiments. Sulfate latex attachment efficiencies were also sensitive to ionic strength increases, but independent of the alkali metal cation type. Given the sensitivity of deposition rates to ionic strength and the lack of evidence to support other repulsion mechanisms, such as hydration or steric forces, the anomalous repulsion is likely to be due to inaccurate assumptions in the EDL expressions.