Effective communication is required for a successful service execution. It can be achieved when both the service consumer and service provider understand the exchanged data message in the same way. Therefore, the specification of the semantics of data message needs to be explicitly defined in the service specification. Six requirements for specifying the semantics of data message are formulated based on the data modeling approach. DEMO is selected to be the methodology for representing the semantics of data message. Since one of the requirements is not completely fulfilled by DEMO; DEMO is extended by using ORM. Finally, the specification of the semantics of the data message of a case study is implemented by using DEMO. It indicates the DEMO usability for specifying the semantics of data messages.