Cells manage oxidative stress from endogenous and exogenous stimuli. Excess oxidative stress causes significant cellular health issues, impacting key biological components such as DNA, membrane, and proteins. Reactive oxygen species (ROS) are a major contributor to oxidative stress, making the measurement of oxidative stress crucial for monitoring cellular health. DCFDA (2′,7′-dichlorofluorescin diacetate) is a widely used probe for detecting oxidative stress in live cells. Once inside the cell, DCFDA is converted to DCFH, which is then oxidized to produce the fluorescent compound DCF. The green fluorescence emitted by DCF under blue light exposure serves as an indicator of oxidative stress. However, DCFDA is a broad oxidative probe and does not provide specificity for different types of ROS. This chapter introduces the use of carboxy-H2DCFDA, a derivative of DCFDA, for monitoring H2O2-induced oxidative stress. Using a fluorometer, the fluorescence of carboxy-H2DCFDA can be measured to provide insights into oxidative stress levels in cells. This approach allows for more precise monitoring and analysis of oxidative stress, enhancing our understanding of cellular responses to oxidative damage.