Abstract Cellular metabolism provides various sources of hydrogen peroxide (H₂O₂) in different organelles and compartments. The suitability of H₂O₂ as an intracellular signaling molecule therefore also depends on its ability to pass cellular membranes. The propensity of the membranous boundary of the endoplasmic reticulum (ER) to let pass H₂O₂ has been discussed controversially. In this essay, we challenge the recent proposal that the ER membrane constitutes a simple barrier for H₂O₂ diffusion and support earlier data showing that (i) ample H₂O₂ permeability of the ER membrane is a prerequisite for signal transduction, (ii) aquaporin channels are crucially involved in the facilitation of H₂O₂ permeation, and (iii) a proper experimental framework not prone to artifacts is necessary to further unravel the role of H₂O₂ permeation in signal transduction and organelle biology.