The use of a normal incidence sub-kev O-2(+) beam in secondary ion mass spectrometry (SIMS) depth profiling minimizes the widths of the transient regions at the surface of a shallow profile in addition to providing a very high depth resolution, At 300 eV we show that the transient width in silicon is <1 nm, and that the ion yields are relatively insensitive to the presence of native oxide. This suggests that ideal conditions for the profiling of very shallow implants are available at 300 eV and below. Nevertheless, the transient signals reflect differences in the thickness of native oxide, as well as differences in primary beam energy, and could, in principle, be used to measure process and wafer age related differences in the top few nm. We present one of the earliest attempts to accurately calibrate the depth in the pre-equilibrium region of a SIMS profile, taking account of the variation in erosion rate.