A formal catalog representation of Cloud services is required for automated fulfillment of user requests on catalog items, sharing of building blocks across service offerings and semantic interoperability between IT platforms. The technologies and tools provided by the Semantic Web community are well suited to achieve this formal representation. In this paper, we give an analysis on the commonality and differences of typical Cloud service offerings and of their catalog actions' mappings to delivery operations. We introduce a new ontological representation that leverages Semantic Web technology to formally model the structure and relationships of these service offerings and their operational processes. Our algorithm to select and order the execution of appropriate delivery operations uses this representation together with a new notion of safe sequences. This algorithm realizes the catalog action requests while ensuring system robustness by following sequencing rules we developed to prevent failures during the fulfillment processes for complex offerings. We evaluate our model on concrete Cloud offerings. © 2011 IEEE.