We study the hydrogen-induced passivation of interface traps in n-GaAs:Si, grown by MBE at 580°C, with a buffer layer grown at 200 to 250°C (LT-buffer). In the as-grown samples, the LT-buffer contains As-precipitates, with a density of 4×1016 cm−3 and a diameter that depends on the LT-buffer growth temperature and takes values in the range 4–8nm. The epilayer/LT-buffer interface in the as-grown samples is characterised by interface related traps which dominate the electrical behaviour of the epitaxial layer. Secondary ion mass spectroscopy profiling of deuterated samples reveals that 2H has a nearly uniform concentration throughout the epilayer, while it accumulates at the interface. Hydrogenation induces a reduction of the interface trap concentration and a significant improvement of the carrier mobility values.