The development of new services by composition of existing ones has gained considerable momentum as a means of integrating heterogeneous applications and realising business collaborations. Services that enter into compositions with other services may have transactional properties, especially those in the broad area of resource management (e.g. booking services). These transactional properties may be exploited in order to derive composite services which themselves exhibit certain transactional properties. This paper presents a model for composing services that expose transactional properties and more specifically, services that support tentative holds and/or atomic execution. The proposed model is based on a high-level service composition operator that produces composite services that satisfy specified atomicity constraints. The model supports the possibility of selecting the services that enter into a composition at runtime, depending on their ability to provide resource reservations at a given point in time and taking into account user preferences.