To establish and implement a workable e-Government, all possible and relevant stakeholders' systems need to be inter-connected in such away that the hardware, software and data are interoperable. Thus, interoperability is the key to information exchange and sharing among the heterogeneous systems. In view of this, the paper introduces the Information Systems Interoperability Maturity Model (ISIMM) that defines the levels and degree of interoperability sophistication that an organisation's Information Systems will progress through. ISIMM focuses more on detailed technical aspects of interoperability that allows data to be exchanged and shared within an information system environment. In this way, it provides the practical means of assessing technical interoperability between information system pairs, groups or clusters and it facilitates a model to measure the maturity and compliancy levels of interoperability information systems.