At hydraulic structures (e.g. weirs, spillways), air-water transfer of atmospheric gases (e.g. oxygen) occurs by self-aeration along the chute and by flow aeration in the hydraulic jump at the downstream end of the structure. In this paper, experimental data are re-analysed and compared with a numerical method to predict the free-surface aeration. Aeration at hydraulic jumps is also examined and compared with existing correlations. Calculations of free-surface aeration and empirical correlations for hydraulic jump aeration are combined to predict the DO content downstream of weirs and spillways. Good agreement between calculations and prototype data is obtained. The results indicate that self-aeration might contribute to a large part of the oxygenation taking place at hydraulic structures for small water discharges. For large discharges, the reduction or the disappearance of free-surface aeration affects substantially the aeration efficiency.