Confocal fluorescence images of live Cos-7 cells with varying levels of mitochondrial H2O2 as visualized using MitoPY1. Cos-7 cells incubated with 5 µM MitoPY1 for 60 minutes at 37 °C and imaged with either MitoPY1 (a), MitoTracker Deep Red and Hoechst (overlay, b), MitoPY1 with MitoTracker Deep Red (overlay, c), or in brightfield mode (d). Cos-7 cells incubated with 5 µM MitoPY1 with 300 µM H2O2 added for the final 40 minutes and imaged with either MitoPY1 (e), MitoTracker Deep Red and Hoechst (overlay, f), MitoPY1 with MitoTracker Deep Red (overlay, g), or in brightfield mode (h) with a 20 µm scale bar.