An emerging concept in cell signaling is the natural role of reactive oxygen species, such as hydrogen peroxide (H2O2), as beneficial messengers in redox signaling pathways. Despite growing evidence, the nature of H2O2 signaling is confounded by difficulties in tracking it in living systems both spatially and temporally at low concentrations. Here we develop an array of fluorescent single-walled carbon nanotubes that can selectively record in real time the discrete, stochastic quenching events that occur as H2O2 molecules are emitted from individual human epidermal carcinoma cells that are stimulated by epidermal growth factor (EGF). We show mathematically that such arrays can distinguish between molecules originating locally on the cell membrane from other contributions. We find that EGF induces 2 nmol H2O2 locally over a period of 50 min. This platform promises a new approach to understanding the signaling of reactive oxygen species at the cellular level.