Institutionalized activities are performed with precise purpose or mission by legitimate role holders, according to the established norms. This is made possible by organizational stock of knowledge which incorporates typification of roles, their obligations and responsibilities, types of actions they perform, and norms governing the execution of activities. The purpose of computerized support to institutionalized activities is to increase their efficiency and effectiveness and, what is of particular importance to this kind of activities, to improve their conformity with norms. The Formal model of activities is created to provide a new communications medium for activity performance, to assist participants in perceiving and understanding social context and how it is affected by activity performance. The paper presents logic- based Formal models of a simple activities type, an activity with recurrence and hierarchically structured activities. They are illustrated by an example.