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Gas emission and the evolution of nutrient composition of pig slurry were evaluated comparing two types of slurry from fattening pigs: with a previous solid separation (SS) and raw slurry (RS). Slurry was obtained at the end of a 19-week fattening period from an experimental pig unit. Six 100 L vessels of slurry were filled, three of them with RS and three of them after the solid-liquid separation (SS). Methane, carbon dioxide, nitrous oxide and water vapor emission were registered during 24-h periods using a photoacoustic monitor. Measurements were conducted once per week during 15 weeks. Samples of slurry were taken fortnightly and analyzed for total solids, volatile solids, total and dissolved Kjeldhal nitrogen, volatile fatty acids (VFA), dissolved chemical oxygen demand (CODd) and pH. Dissolved chemical oxygen demand, VFA and carbon dioxide emission increased on week 3 of the study, especially in RS, indicating an important acidogenic and acetogenic activity during this time. Methane emission was minimal during the first 6 weeks of storage and reached the maximum levels between week 9 and 12, being 0.099 mg h-1gCOD-1initial in SS and 0.152 mg h-1gCOD-1initial in RS (p<0.001). Thus, from week 9 of storage period, decomposition of organic matter was mainly due to methanogenic bacteria. Between treatments, RS showed better anaerobic conditions than SS and thus, greater methane and nitrous oxide emissions.