The removal efficiency of particlcs in deep bed filtration depends on both transport and attachment mechanisms of particles. The particle attachment mechanisms depend mainly on chemical characteristics of aqueous phase, suspended particles and filter media. In this study, two different suspensions were used to study the effect of ionic strength on the particle removal efficiency. The ionic strength of suspensins was varied by adding a known amount of sodium chloride. The removal efficiencies were related to ionic strength and zeta potentail. The clean filter bed efficiency and removal efficiency during the ripening period for various strengths were quantitatively related to the two model coefficients iuring the ripcning pcriod l`vr various strcngths werc quantitativcly relalcd to thc two model coethcicnls appering in the O'Melia-Ali model. These model coefficients represent the filter medium-particle and particle-particle attachments. This type of quantificaton of the effect of ionic strength in terms of filter efficiency is very useful in filter design and operation control.