Methane (CH4) and carbon dioxide (CO2) emissions from Brazilian reservoirs were assessed. Point measurements were made during 1998 and 1999 (using inverted funnels for bubbles and air and water concentration gradients for diffusion) in the 559 km2 Samuel reservoir, which was initially flooded in 1988, and the 2430 km2 Tucuruí reservoir, which was flooded in 1984, and the data were evaluated with respect to historical measurements in other Brazilian reservoirs. Bubble emissions of CH4 were higher in Samuel (ranging from 2 to 70 mgCH4 m−2 day−1) than in Tucuruí (ranging from 0·5 to 30 mgCH4 m−2 day−1), with the highest values occurring the shallowest regions in each reservoir. CH4 from diffusion for the Tucuruí reservoir ranged from 5 to 30 mgCH4 m−2 day−1, which is lower than that for the Samuel reservoir, which ranged from 10 to 80 mgCH4 m−2 day−1. The smaller emissions in Tucuruí compared with Samuel are attributed to a larger depletion in the source organic material that was present when the reservoir was filled. The CO2 concentration was similar for each reservoir, and ranged from 1000 to 10 000 mgCO2 m−2 day−1. Copyright © 2003 John Wiley & Sons, Ltd.