The lack of methodological support for reuse has been identified as one of the major causes why software developers cannot take full advantage of reuse pay offs such as software productivity, quality, and cost improvement. There is,therefore, a need for explicit definitions about how to practice reuse as part of the development process. These definitions include models and properties of reuse techniques which can improve understanding and provide more guidance for the software developers that want to adopt reuse. In this paper we provide a model and properties related to are use technique called separation of concerns that can be applied in object-oriented design. The formal model define show to combine classes or components dealing with different concerns in object-oriented design so that separation of concerns is achieved. The model is presented as a design relationship which is characterized by a set of properties. These properties can be seen as a semantics for gluing object-oriented components with separation of concerns in mind. We illustrate the model by presenting some of the typical properties that describe the combination. We also show how the model can be used in user interface design.