The textbook approach to interfacial excess quantities, due to Gibbs, is valid only at fixed volume. In the present work we develop an alternative approach to interfacial excess quantities, valid at fixed mass. For solid–solid interfaces this has several advantages over the Gibbs treatment such as (a) non-zero interfacial excess volumes appear naturally in the treatment, (b) analogues of the bulk-phase Maxwell relations are readily obtained, and (c) formulae for the pressure and temperature dependence of the interfacial tension are derived in a transparent way.