The electron spin polarization (ESP) of the L-gap surface states on Au(111) is investigated theoretically by means of first-principles electronic structure and photoemission calculations. The surface states show a large spin-orbit induced in-plane ESP which is perpendicular to the in-plane wave vector, in close analogy to a two-dimensional electron gas with Rashba spin-orbit interaction. The surface corrugation, i.e., the in-plane asymmetry of the surface potential in the 1x1 unit cell, leads to a small ESP component normal to the surface. The surface-state ESP can be probed qualitatively by spin- and angle-resolved photoelectron spectroscopy, provided that the initial-state ESP is retained in the photoemission process and not obscured by spin-orbit induced polarization effects. Relativistic photoemission calculations provide detailed information on what photoemission setups allow one to make conclusions from the photoelectron ESP about that of the surface states.