Long-term data from three full-scale trickling filters installations accomplishing combined carbon oxidation and nitrification are presented. Analysis of these data indicates that the oxidation of organic matter and ammonia-nitrogen in the trickling filter can be characterized by a single parameter which is calculated as the trickling filter organic loading (expressed in terms of BOD5 or biodegradable COD) plus the ammonia-nitrogen oxidized converted to oxygen equivalents using the conversion factor 4.6 kgO2/kg NO3-N. This parameter, which is referred to as the volumetric oxidation rate and is expressed in the units of kgO2/m3-day, characterizes trickling filter performance over a wide range of process loading and effluent quality conditions. The method of analysis presented in this work differs from the one traditionally used to characterize combined carbon oxidation and nitrification in trickling filters, and its suggests that oxidation may be oxygen transfer limited throughout such a trickling filter.