In the last few years heterostructures consisting of a-C:H on top of crystalline silicon have revealed a number of interesting properties for electronic applications, for example, as a photodiode (see Ref. 1) or, very recently, as cold cathode electron emitter (see Ref. 2). For those applications the interface properties of the heterostructure play a key role. In this article the electronic and chemical nature of the interface between plasma deposited a-C:H in the form of diamondlike carbon (DLC) and hydrogen terminated silicon (111) was determined by in situ core level and valence band photoemission spectroscopy. The quantitative analysis of core level shifts and intensities indicate that both, an interface layer of mixed composition and a pure a-C:H layer, grow simultaneously with relative growth rates of 2:1 during the initial stages of the interface formation. The interface layer is first carbon rich and then approaches stoichiometric composition which is reached at an interface thickness of 18 Angstrom. Then a reactive SIC interface is completed and acts as a diffusion barrier which prevents further extension of the interface region when the deposition of a-C:H proceeds. Aside from an acceptor passivation of the Si substrate, due to H atoms from the plasma, no overlayer induced band bending is measured. The valence band offset between Si and DLC is determined as 0.7+/-0.1 eV with the valence band edge of DLC below that of silicon.