This paper reports the results obtained in a study of the aerobic stabilization of sludge in a laboratory-scale reactor. A variety of parameters were measured including: physicochemical (pH, dissolved oxygen, temperature, volatile and total solids, chemical oxygen demand and hydrogen sulfide production); microbiological (fecal coliforms, Escherichia coli. viable biomass, and the relationship between active and total cells); and measurements of enzymatic activity (oxygen uptake rate, dehydrogenase activity and esterase activity). From the results, it may be concluded that the traditional physicochemical and microbiological parameters present a series of problems. which detract from their usefulness. The enzymatic parameters, dehydrogenase activity (primary metabolism) and esterase activity (secondary metabolism) are better able to characterise the process; and the quotient between these two variables may be used to estimate the degree of endogenesis and, consequently, the degree of stability of the aerobic sludge digestion. In addition. these techniques are swift and simple to employ. (C) 2004 Elsevier Inc. All rights reserved.