Abstract: A new reference electrode assembly is provided that includes a hydrogen ion specific electrode which is encapsulated in a polymer matrix. The bulk of the polymer matrix has an open cell structure within which a liquid is contained as the electrode potential producing agent, while its surface is such that it forms a barrier preventing a free and open connection to the environment of the reference electrode. The reference electrode assembly is especially suitable for in vivo measurements, and it may be designed so that it is substantially insensitive to pH variations of the solution being measured by the instrument of which the reference electrode assembly is a part.