A Cameca IMS-4f secondary ion mass spectrometry instrument has been applied to the study of the diffusion of ultralow energy B implants in crystalline silicon. Several analyses on sub-keV B implants have been performed by using low energy O-2(+) beams both before and after thermal annealing. The limits and the accuracy of the technique are discussed. It is shown that a 1.5 keV beam provide he depth resolution needed to accurately characterize, beyond the equilibrium depth (similar to 5 nm), a 500 eV B implant. This measurement protocol provides at the same time a significantly low detection limit (1X10(15) at/cm(3)) and a very fast sputter rate (25 nm/min), necessary to characterize deep diffused profiles. Several artifacts are discussed, with emphasis to these affecting the tail region of the profiles.