The reduction by dithionite ion (in excess) of methemerythrin-anion adducts, Hr+X-, to deoxyhemerythrin, Hr°, has been examined at 25° and pH 6.3 and 8.2. The results accord with the scheme: {A figure is presented} With X- = Br-, HCO2-, CNO-, and F-, k2[SO2-] 〉 k1[X-], and the pseudo first-order rate constant, kobs (=k-1), is independent of [X-] and [S2O42-]. Only with X- = NCS- is k2[SO2-] ∼ k1[X-] and kobs = a[S2O42-] sol1 2 (b[NCS-] + [S2O42-] sol1 2)-1. Values at pH 6.3 of k-1 (sec-1) and k1 (M-1 sec-1), obtained by anation and anion displacement reactions, are 2.3 × 10-3, 1.6 × 10-2 (Br-); 1.5 × 10-3, 1.2 × 10-2 (HCO2-); 1.3 × 10-4, 0.52 (CNO-) and ∼2 × 10-4, 3.3 × 10-3 (CN-, pH 7.0). Values of k-1 from reduction and displacement methods are in good agreement with each other. The value of k2 (1.6 × 105 M-1 sec-1, pH 6.3) is somewhat smaller than that for reduction of the met form of hemoproteins. There is only a small effect of pH on rates. Direct reduction of Hr+CN- does not occur, in contrast with Mb+CN-. © 1979.