After discussing the features of a differential ISFET/REFET measuring concept, the published attempts to construct a proper REFET are summarized. It is concluded that the present REFETs are based upon the addition of a blocking polymeric layer to the gate surface of an ISFET, but that this approach fails with respect to the required insensitivity to ionic strength variations as well as with respect to the electrical stability. As a solution to these problems, this paper describes the development of a REFET concept that is based on the chemical attachment of a non-blocking polymeric layer. Characterization methods of these layers with respect to the electrical as well as the chemical behaviour are given and discussed. Finally, the experimental results of an acrylate/polyHEMA-REFET are shown in a differential ISFET/REFET system.