The kinetics of phosphate (P) during a treatment of hemofiltration or hemodiafiltration follows a simple exponential law, and P in 90 min reaches a stationary level that remains constant until the end of the treatment. The theoretical compartmental model cannot be estimated by a dialytic treatment, but a simpler diffusive model may be considered. Once the stationary level is reached, the net flux of P from the inaccessible compartments to the accessible compartment may be estimated via the parameters obtained in the exponential model. Hemodiafiltration seems more efficient than hemofiltration in determining P clearance. Thus an efficiency index that accounts for the period in which P is eliminated from the accessible compartment has been formulated.