Enterprise Services have been proposed as a more business-friendly form of web services which can help organizations bridge the gap between the IT capabilities and business benefits of Service Oriented Architecture. However up until now there are almost no methodologies for creating enterprise services, and no lists of definite criteria which constitute a good enterprise service. In this paper we present a model which can aid Service Oriented Architecture designers with this by giving them a set of researched criteria that can be used to measure the quality of enterprise service definitions. The model and criteria have been constructed by interviewing experts from one of the big five consultancy firms and by conducting a literature study of software development lifecycle methods and Service Oriented Architecture implementation strategies. The results have been evaluated using a quantitative survey and qualitative expert interviews, which have produced empirical support for the importance of the model criteria to enterprise service design. The importance of business ownership and focusing on business value of enterprise services is stressed, leading to suggestions of future research that links this area more closely with Service Oriented Architecture governance, Service Oriented Architecture change management, and Business Process Management.