After O2 ions, Cs is the second most important probe in high-sensitivity SIMS depth profiling analysis due to the achievable high (negative) secondary ion yields. However the depth distribution of Cs is not very well known. We have investigated the buildup of the (internal) Cs concentration profile in Si following 5 keV Cs bombardment at 45° for various fluences ranging from 1014 to 3 × 1016 ions cm2. The implants were carried out in an UHV low-energy implanter after thermal removal of the native oxide layer. Ex-situ RBS analyses were carried out to establish the retained Cs dose as a function of fluence. These showed that saturation conditions are reached after a fluence of 7 × 1015 ions cm2 (areal density 1.7 × 1015 at. cm2). The near-surface Cs distribution at saturation was analyzed in-situ by high depth resolution MEIS (< 7 Å). 10% of the Cs concentration was found to be segregated within the first 20 A. SIMS studies of the Cs implant at saturation using O2 +beams were undertaken after in-situ capping of the implant with a 100-150 Å ion beam deposited isotopic 28Si layer, by which depth profile distortion is minimized. Both MEIS and SIMS results show the Cs distribution to extend to beyond 150 Å, far in excess of the TRIM predicted projected range. The experimentally obtained profiles have been compared with extensive IMPETUS simulations. Although the FWHM of the experimental and simulated profiles are similar (70 Å), the poor agreement on the maximum range of the Cs in Si suggests that a more complex, variable Cs volume model needs to be adopted and/or that additional effects such as segregation and radiation enhanced diffusion need to be incorporated in the model.