This paper describes an experimental investigation into the effect of backwash rate, type of coagulant, degree of clogging and accumulation of residual deposits (not removed by backwash) on the efficiency of fluidized bed filter backwash in laboratory scale filters treating a natural raw water coagulated with either alum or a cationic polymer blend. Backwash efficiency was defined in terms of the mass of floc retained in the filter after backwashing. Small variations in the raw water characteristics (manifested as variations in raw water turbidity, temperature, pH, rate of headloss development and turbidity removal efficiency) within each set of experiments appeared to affect the efficiency of backwash in addition to the parameters varied deliberately. Stepwise linear regression was used to determine which of several possible filtration and backwash parameters were the best predictors of backwash performance. Backwash rate, filter run time, rate of headloss development and mass of residual deposits accumulated during previous runs were found to be the best predictors of backwash efficiency for any given filter cycle. Floc deposits appeared to become more difficult to remove the longer they remained in the filter, while rate of headloss development appeared to provide some indication of floc adhesiveness for filter runs of similar length. The efficiency of detachment of freshly deposited floc appeared to increase as the mass of residual deposits and mudballs in the filter increased. The simple linear correlations developed here provide an important first step towards developing and calibrating a more comprehensive model of backwash efficiency that could be easily integrated with existing models of filtration. While this investigation was limited to fluidized bed backwash without air scour or surface wash, the general approach is equally applicable to the more technologically relevant case of backwash with auxiliary wash. Includes 18 references, tables, figures.