The anaerobic bioavailability of two types of solid slaughterhouse waste (poultry and pig wastes) was studied after the application of thermal pretreatment. Both wastes were characterized by different proportions between fats, proteins and carbohydrates. The thermal pretreatments selected (70ºC for 60 min, and 133ºC and 3 bars for 20 min) were included in the European regulations 1774/2002 & 92/2005 for higienization purposes. These pretreatments had a remarkable effect on the solubilization of organic matter and on protein hydrolysis. No negative effects in methane or biogas yields were found in both pretreated wastes, but the bioavailibity of organic matter after the pretreatment depended on the composition of each waste, basically the presence of carbohydrates which could react with protein hydrolysis products (amino acids) giving slowly or not anaerobically biodegradable products (Maillard compounds).