Yields of hydrogen and nitrogen have been measured for acidified solutions of potassium ferrocyanide containing nitrous oxide illuminated with 2537 Å light. From the effect of decreasing pH and added nitrate ions which diminish ϕ (N 2 ) and the effect of isopropanol which acts as an H atom scavenger it is concluded that (i) k ( e ¯ aq. + HFe(CN) 3– 6 ) / k ( e ¯ aq . + N 2 O) = 0·25 ± 0·1 at μ = 0 and increases with increasing μ ; (ii) k (H + NO ¯ 3 ) / k (H + (CH 3 ) 4 CHOH) = 0·47 ± 0·05; (iii) k (H + Fe(CN) 3– 6 ) / k (H + (CH 3 ) 2 CHOH) = 72 ± 7 and is independent of μ ; (iv) the protonated ferrocyanide ion, HFe(CN) 3– 6 absorbs 2537 Å light and undergoes decomposition according to either ( a ) HFe(CN) 3– 6 + hv → H + + Fe(CN) 3– 6 + e ¯ aq. or (b) HFe(CN) 3– 6 + hv → H + Fe(CN) 3– 6 which is opposed by geminate recombination and such that ϕ a + ϕ b < 0·66, the quantum yield of Fe(CN) 4– 6 + hv → Fe(CN) 3– 6 + e ¯ aq. . The implications of these results are discussed.