The selection of a suitable middleware platform is a critical task in developing modern software systems. It is critical to fulfil the systems quality requirements, e.g. availability. A systematic selection should be done as early as possible within a development project, i.e. on the architectural level. Up to now no adequate method for selecting a middleware product exists, thus often ad-hoc decisions are made. As a solution to this problem, this dissertation proposes the MidArch Method based on a pre-study of usages of architectural styles. The method is based on modelling middleware platforms by middleware-oriented architectural styles, evaluating candidate architectures that conform to these styles, as well as mapping these evaluations to the platform level. Two case studies using industrial systems are presented. Furthermore, the thesis contains contributions in the areas of software architecture foundations and the definition of architectural styles and patterns.