Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for the O-like ion S IX. The configurations used are 2s 22p 4, 2s2p 5, 2p 6, 2s 22p 33s, 2s 22p 33p, and 2s 22p 33d giving rise to 86 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 25, 50, 75, 100, and 125 Ry. Excitation rate coefficients are calculated by assuming a Maxwellian electron velocity distribution at several electron temperatures in the 5.6 ⩽log Te (K)⩽6.2 range, where S IX is formed. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10 8-10 14 cm -3. Relative spectral line intensities are calculated. Proton excitation rates among the lowest three levels have been included in the statistical equilibrium equations. The predicted S IX line intensities are compared with SUMER (SOHO) observations of the quiet Sun.