We present a theoretical study on the impact of an interface on the FRET efficiency of a surface-bound acceptor-donor system. The FRET efficiency can be modified by two effects. Firstly, the donor's electromagnetic field at the acceptor's position is changed due to the partial reflection of the donor's field. Secondly, both the donor's and the acceptor's quantum yield of fluorescence can be changed due to the interface-induced enhancement of the radiative emission rate (Purcell effect). Numerical results for a FRET-pair at a glass-water interface are given.