In this work, the conditions for measuring the specific denitrifying activity (SDA) of sludges were established as simple batch tests based on the measurement of N-2 production. The effects of carbon to nitrogen ratios (C/N = 1.3 and 2.0), sludge concentrations (0.35, 0.73, 1.5 and 3.0 g volatile suspended solids (VSS) 1(-1)) and shaking (0-180 rpm) on SDA were studied by means of the application of two factorial designs. A very simple method (Method A), based on the displacement of an alkaline solution by the N-2 generated, was used. In the 0.35-0.73 g VSS 1(-1) range, the SDA was strongly affected by variable VSS, but not in the range 1.5-3.0 g VSS 1(-1). The optimum conditions to obtain the SDAs, evaluated by means of a statistical tool, were a C/N = 1.3, 1.5 g VSS 1(-1) and shaking at 180 rpm. Method A was statistically compared to a second method (Method B), based on the measurement of partial pressure and composition of the gas produced in the same conditions.