Interface and surface passivation of thin layers of III-V compound semiconductors is a key enabler of many technological applications spanning from nano-electronics to nano-photonics. The interaction between thin amorphous Si (a-Si) layers and clean, group-III-rich reconstructed In0.53Ga 0.47As interfaces is studied by combining high-resolution synchrotron radiation x-ray photoelectron spectroscopy and time-dependent surface photovoltage (SPV) experiments. From 0.6 to 2.4 monolayers (ML) of a-Si are deposited on non-intentionally doped, p- and n-type In0.53Ga 0.47As. For each deposition step, the observed surface and interface chemistry is directly correlated to the measured surface Fermi level position, band bending and SPV. Group-III-reconstructed In0.53Ga 0.47As surfaces are observed to be intrinsically unstable against reaction with Si and two different instability regimes have been identified. First, for low deposition temperature, Si reacts strongly and intermixes with the In0.53Ga0.47As surface inducing In and Ga out-diffusion even at a sub-monolayer amount. For 2.4 ML of a-Si, a net positive interface charge of 1.24 × 1012 #/cm2 and a band of defects close to the conduction band are detected. For post-annealing at temperatures lower than 380 °C, the interface rearranges. At temperatures higher than 380°C, out-diffusion of As in the a-Si is found to be the main instability driver. © 2014 IOP Publishing Ltd.