The organization-based access control (OrBAC) model is an access control model that helps evaluate the security policies of organizations. OrBAC affords a high degree of expressiveness and scalability. The model, however, does not readily express integrity constraints. Integrity is one of the most important properties for critical infrastructure systems, mainly due to their criticality and low tolerance of corruption and alterations. This paper describes an extension of OrBAC, called Integrity-OrBAC (I-OrBAC), which models integrity attributes associated with critical infrastructure systems. I-OrBAC facilitates the modeling of multiple integrity levels to express the requirements of different critical infrastructure organizations. An example security policy is presented to demonstrate the expressiveness of the model.