SIMS and medium energy ion scattering (MEIS) have been applied to the characterization of ultra shallow distribution of arsenic in silicon obtained by ion implantation at 1 and 3 keV and successive annealing at low temperature (lower than 700 °C). In case of heavy elements in light matrices, the MEIS ultimate depth resolution and its ability to obtain quantitative results from first principles result in a good reference for SIMS depth profiling. The comparison of the results obtained by the two techniques allows to discriminate among different SIMS quantification processes in order to individuate the best in terms of accuracy in the initial transient width and at the SiO2-silicon interface: the simple normalization of 28Si75As- curve to the average of 28Si2 - results in the best agreement between SIMS and MEIS in the surface region. Moreover SIMS profile of the 3 keV as implanted sample resulted 1.9 nm shallower than correspondent MEIS profiles whereas samples annealed at either 650 °C or 700 °C for 10 s show a good alignment of the As segregation peak at the SiO2/Si interface. The sample annealed at 550 °C for 200 s shows a reduced shift between SIMS and MEIS measured As peak: a possible effect of the residual amorphous layer on the sputtering rate is pointed out as responsible of these different shifts.