Although electronic information systems (IS) that act as main sources of information in an ecosystem of digital systems are important in the success of IS interoperability, the conditions and challenges of source systems that affect IS interoperability have not received enough attention in the body of existing scientific knowledge. Using the System Theory, this paper relies on empirical evidence from two IS interoperability case studies to argue that the interoperability of electronic information systems can only succeed if due attention is paid to the individual systems that are involved in an IS interoperability project. Among other things, it was found that IS interoperability can only be achieved if the individual systems meet the requisite conditions, and that an environment surrounding a source system could substantially impact its ability to be interoperable with other systems.