Several con trol and monitoring applications are implemented using commercial off the shelf (COTS) PLCs that were not necessarily developed according to nuclear standards. The UK nuclear regulatory regime requires that a safety case be developed to justify and communi cate their safety. Typically, the assessment of COTS components has been done with a focus on standards compliance compliance to accepted practice was deemed to imply adequate safety. However, there may be a number of difficulties with justifying COTS pr oducts related to limited knowledge of the internal structure of the components or their development processes, especially when the supplier of the PLC platform is not willing to provide the necessary information to complete a compliance case. This paper d escribes a claim based approach to the justification of COTS PLC components using Cogs , developed in a project funded by the UK nuclear industry The approach ? focuses on the behaviour of the system rather than on the process followed to develop the PLC pla tform ? structures the justification around behaviour attributes (such as functionality, performance and reliability) and considers them in terms of the application and/or platform ? uses information about the platform that is likely to be publicly available f rom the supplier