In order to estimate the expectable enhanced biological phosphorus removal of a wastewater, two parameters are determined: the volatile fatty acids, and the ability to produce them in anaerobic conditions. Because volatile fatty acids are sensitive to modification after sampling, grab samples are mainly characterized. The aims of the present work are to investigate the hourly variations of these two parameters for wastewater, and to study the ability of a 24h-composite automatic sampler to provide them. The consequences of the oxygen input, acidogenesis start-up, and anaerobic consumption were quantified. Two conventional composite sampling techniques were compared leading to a 15% difference. The investigations carried out for two wastewater have shown significant hourly variations around the daily average value with concentrations 3 times higher during 3 hours long.