Long-term usability of software systems is becoming an economically relevant issue. In particular it is important to assess it at an early stage in the development process. In this paper a method for such an assessment is proposed. It is based upon the calculation of a "long-term-useability-index" from a number of defined properties of the modules of the software system under development. There are two versions of the method, that differ in the number of properties to be taken into account, the effort to be invested and the precision of the result. First tests have shown the viability of the method.