Cleaning is done in industry for different purposes: to maintain industrial equipment and work areas; to remove contaminants such as dirt and process materials from the interior of process equipment; and to prepare surfaces before the next stage in a process. Cleaning tools and removable parts also require cleaning. As a result of these activities solvents containing volatile organic compounds (VOCs) are evaporated into the atmosphere, contributing to the air quality problem. A number of solvents used are classified as hazardous air pollutants in Section 112 of the Clean Air Act Amendments (CAAA). The purpose of this paper is to discuss a methodology to determine accurate VOC emissions from cleaning. It is founded on the concept of a 'unit operation system (UOS),' which was developed to define the emission streams that need to be considered for a material balance. The focus, for the purpose of the material balance, is the 'unit operation' being cleaned.