The formation of ripples on Si(100) by O+2 sputtering at an angle of incidence of 40° and energies from 1 to 9 keV has been studied using secondary ion mass spectrometry and scanning electron microscopy. At 1 keV no ripples are observed. Between 1.5 and 9 keV ripples are observed oriented perpendicular to the ion direction with average wavelengths that increase, from ∼100 to 400 nm, approximately linearly with O+2 energy. Two-dimensional fast Fourier transforms of secondary electron images are used to investigate the frequency distribution of the ripples. For the conditions studied, the distributions of frequencies appear approximately Gaussian. At 1.5 keV, the wavelength and growth rate with sputtered depth are independent of flux for fluxes from 15 to 150 μA/cm2. Accompanying ripple formation are changes in secondary ion yields. The changes occur abruptly at depths that increase, from ∼0.2 to 5.6 μm, with O+2 energy. In contrast, sputtering with Ar+ at 1.5 and 7 keV to depths 5–10 times those that produce ripples with O+2 produce no observable topography. These results are discussed using several existing theories for ripple formation and growth. Ripple growth and the variations in secondary ion yield are modeled by accounting for the change in local angles of incidence as the ripples grow. This model describes well the variation in secondary ion yield assuming an exponential growth rate. Ripple formation is discussed in terms of a balance between roughening (by sputtering-induced surface stress and by the dependence of the sputtering yield on surface curvature) and smoothing (by both diffusion and ion mixing). Variation in ripple wavelength with energy is not simply explained by these theories. Surface smoothing by cascade ion mixing can, however, make the wavelength, as observed, independent of ion flux. Finally, the possibility of formation of ripples by phase separation within the SiOx surface layer is discussed.