An upflow velocity of 0.21 m h(-1) was optimal to minimize the effect of organic shocks (from 6 to 30 kg COD m(-3) d(-1)) when operating an upflow anaerobic filter for the treatment of an oleic acid-based effluent (50% w/v COD). This value represented the transition between a mass transfer controlled regime and a kinetic regime. Under hydraulic shock loads, an increase in upflow velocity from 0.3 to 0.4 m h(-1) decreased the removal efficiency from 68 to 51%.