Electron impact collision strengths, energy levels, oscillator strengths and spontaneous radiative decay rates are calculated for S X. The configurations included are 2s22p3, 2s2p4, 2p5, 2s22p23s, 2s22p23p, and 2s22p23d giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 25, 50, 75, 100, and 125 ryd. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved. Proton excitation rates between the levels of the ground configuration have also been calculated and included in the statistical equilibrium equations. The predicted S X line intensities are compared with SUMER observations of the quiet Sun.