Oxygen can cause serious corrosion and deposition problems in boiler feedwater systems. As a result, dissolved oxygen is an important on-line chemistry measurement in all fossil fuel, combined cycle, and other power plants. Plant-specific operating conditions and guidelines dictate where and how frequently to monitor as well as the range, accuracy, and overall performance requirements for oxygen measurement. In each case, a suitable on-line analyzer is essential to verify protection and to reliably indicate when corrective action is required. A basic understanding of oxygen corrosion and dissolved oxygen measurement practices and theory, analyzer selection criteria, installation guidelines, and optimization and verification techniques will ensure reliable and cost-effective operation and response.