Abstract Following mechanical separation of pig slurry, two pilot-scale reactors were used to treat aerobically the liquid fraction with a 4 day residence time and an operating temperature of 35°C. In one reactor redox potential (RP) was controlled to between −145 and −45 mVE cal and, in the other, dissolved oxygenn (DO) to between 1–2 mg O 2 litre −1. Unseparated and separated slurries and two aerobically-treated (RP and DO) slurries were applied to grassland plots at 8 litres m −2 and a system of small wind tunnels used in the collection of odorous air samples and in the measurement of ammonia volatilisation. Odour measurements were conducted by dynamic dilution olfactometry. Both aerobic treatments reduced the total odour emission over 52 h by 55% compared with unseparated, untreated slurry, whilst separation alone gave a 26% reduction. However, during the first 2 h after spreading, when the rate of emission was highest, odour emission was 41·2, 29·8 and 21·6 odour units s −1 for unseparated,separated and RP-treated slurry, respectively, but only 4·0 odour units s −1 for DO-treated slurry. Aerobic treatment also reduced odour intensity and odour offensiveness. Both aerobic treatments followed by storage increased slurry pH, which led to an increase in the total loss of N through ammonia volatilisation after spreading on land.