The presence of free O–H groups at the water-air interface is essential for the uptake of gas molecules and surface chemical reactions, but their microscopic structure remains elusive. Combining polarization-resolved surface-specific spectroscopy with simulations, we unveil the heterogeneous structure of the free O-H group. Free O-H groups originate predominantly from well-structured water molecules that sit, on average, on the liquid side below the Gibbs dividing surface rather than from the more disordered molecules above the Gibbs dividing surface that protrude furthest into the vapor.