Several animal feed additives are authorised in the European Union as acidity regulators; some of them have potential benefits beyond the digestive effects including the reduction of the emissions of ammonia from the effluents subsequently produced. Benzoic acid (C6H5COOH) is only provisionally authorised from May 2003 for a period of four years. After being absorbed in the small intestine and combined with glycine, it is transformed into hippuric acid (C6H5CONHCH2COOH) and then eliminated through the kidneys with the first consequence of decreasing the pH of urine. The reduction of dietary protein content is also known to reduce urinary pH and ammonia volatilization. The aim of this study was to evaluate the combined effects of reduced protein supply and supplementation with benzoic acid on ammonia volatilization. Sixteen fattening pigs were fed with 4 experimental diets, which differed in their crude protein content, (13 or 18 %), and in the addition of benzoic acid (0 or 1%). All the excreta from the 4 pigs in each dietary treatment were collected over a 9-day period. The weight and composition of urine and faeces were determined including urinary pH. Five kilograms of slurry prepared from the mixing of the excreta were placed in a laboratory pilot scale system designed to measure ammonia emission; this equipment allowed the simultaneous measurement of ammonia emission from 12 cells. Changes in slurry composition were determined by mass balance calculations and ammonia emission was measured during two storage trials of 95 and 238 hours duration. Potential of ammonia volatilisation after spreading was measured using the wind tunnel technique for the slurry from the two 13 % crude protein diets, with or without benzoic acid supplementation (3 replicates of five days each). The pH of the urine varied from 9.32 (18% protein, 0% C7H6O2) to 7.94 (13% protein, 1% C7H6O2). The pH of the fresh slurry (obtained from the mixing of urine and faeces) for the same treatments varied from 8.77 to 7.59, respectively, and it decreased to 7.58 and 6.77 after 10 days of storage. Average weight loss of slurry during the two volatilisation trials was 2.27 and 6.12 %, respectively and can easily be explained by the evaporation of water. The mass balance of nitrogen components varied from 94.74 % to 99.65%. Total nitrogen (T-N) losses varied from 4 to 16 % for all experimentations. Volatilised ammonia trapped in acid bubbling tubes varied from 0.65 to 3.90 grams N per cell over all the experiment. Over the 238-hour trial we observed a difference in ammonia emission of 61 % between the high ammonia potential diet (18 % protein and 0 % benzoic acid) and the low potential diet (13 % protein and 1 % benzoic acid), for the same amount of stored slurry. Volatilisation of ammonia after the spreading of slurry from animals with the 13 % protein diets was very small. There was no significant differences between the two slurries. Such results reflect the low pH values of spread slurries, respectively 7.53 and 6.77 for 13 % protein diet at 0 and 1 % benzoic acid.