Holonic Manufacturing Systems are a response solution for the emer-gent need of flexible, reactive and productive manufacturing systems. This pa-per relies on PROSA, a classical holonic referencearchitecture, whichmakes use of a product specification, a process specification and a means to determine a resource's productionabilitiesand capacity, but does not define a specific me-thod for representing such.This paper proposes an approach to define a prod-uct's process specification model that integrates the principles and advantages of Service-oriented Architectures, Petri-Nets and Product Families. Then, are-definition of the basic holons is given to have a glimpse on a possible exploita-tion of this new approach, together with a short-term forecasting strategy, for the flexible orchestration of workflows. Finally, it is shown how the proposed-product's process specification model enhances the HMS's flexibility,reactivity and productivity giving rise to a Service-oriented HolonicManufacturing Sys-tem.