This paper reports the first example of a reductive C(sp³)–C(sp³) homo-coupling of benzyl/allyl halides in aqueous solution by using H₂ as an electron source {turnover numbers (TONs) = 0.5–2.3 for 12 h}. This homo-coupling reaction, promoted by visible light, is catalysed by a water-soluble electron storage catalyst (ESC). The reaction mechanism, and four requirements to make it possible, are also described.