For a couple of decades, process quality has been considered as one of main factors in the delivery of high quality products. Multiple models and standards have emerged as a solution to this issue, but the harmonization of several models in a company for the fulfillment of its quality requirements is no easy task. The difficulty lies in the lack of specific guidelines and in there not being any homogeneous representation which makes this labor less intense. To address that situation, this paper presents an Ontology of Process-reference Models, called PrMO. It defines a Common Structure of Process Elements (CSPE) as a means to support the harmonization of structural differences of multiple reference models, through homogenization of their process structures. PrMO has been validated through instantiation of the information contained in different models, such as CMMI-(ACQ, DEV), ISO (9001, 27001, 27002, 20000-2), ITIL, Cobit, Risk IT, Val IT, BASEL II, amongst others. Both the common structure and the homogenization method are presented, along with an application example. A WEB tool to support the homogenization of models is also described, along with other uses which illustrate the advantages of PrMO. The proposed ontology could be extremely useful for organizations and other consultants that plan to carry out the harmonization of multiple models. Keywords: Harmonization of multiple models and standards; homogenization; mapping; integration; ontology, processes.