<p>High-velocity jets entrain air as they fall through the atmosphere, enhancing energy dissipation and thus being commonly used in high dams. However, the detailed evolution of self-aeration along such free-falling jets–from non-aerated to fully aerated–remains poorly understood. This study presents systematic large-scale experiments on free-falling jets, using a phase-detection probe, to examine their lateral aeration diffusion under various initial water depths and velocities. The test results show that the streamwise evolution of air entrainment can be divided into three stages–undeveloped, developing, and fully developed–and that the aerated flow region extends with increasing initial Froude number. It was also found that the average sectional void fraction is governed primarily by the initial geometry, e.g. the initial water depth and the trajectory length, rather than by the initial velocity. Moreover, the jet break-up length exceeds eight times the initial water depth and increases with the Froude number following a power law. A turbulent diffusion coefficient is introduced to fit the complex void fraction distribution into a single metric, which is proportional to the initial water depth and velocity. Based on these insights, predictive equations for the void fraction distribution and the average sectional void fraction under various initial flow conditions are proposed. These findings advance the fundamental understanding of air-water two-phase flow in high-velocity jets.</p>