The relative amounts of fats, proteins and carbohydrates in wastewaters from dairy industries cause problems during their anaerobic treatment. The anaerobic biodegradability of two synthetic wastewaters, one rich in fats (chemical oxygen demand (COD) ratio; Fats/Proteins/Carbohydrates: 1.7/0.57/1) and the other with a low fat content (COD ratio; Fats/Proteins/Carbohydrates: 0.05/0.54/1) was studied in samples with total COD ranging from 0.4 to 20 g/l, There were no problems of sludge flotation and the maximum biodegradability and methanisation were obtained when operating with wastewaters in the range of 3-5 gCOD/l. The intermediates of fat degradation (glycerol and long chain fatty acids) seemed not to reach concentrations high enough to affect the process. The anaerobic biodegradation of fat-rich wastes was slower than carbohydrate-rich wastes due to the slower hydrolytic step of fat degradation which prevented the accumulation of volatile fatty acids (VFAs) and favoured the overall process. Carbohydrate-rich wastewater degradation produced free ammonia (FA) at concentrations near to inhibitory levels (62.2 mg FA/l), but in this case, ammonia production facilitated regulation of fall in pH caused by of the accumulation of VFA.