THE structural end-points of haemoglobin’s transition from its low-oxygen-affinity (T) to high-oxygen-affinity CR) state, have been well established by X-ray crystallography(1-7), but short-lived intermediates have proved less amenable to X-ray studies, Here we use chemical crosslinking to fix these intermediates for structural characterization. We describe the X-ray structures of three haemoglobins, alpha(2) beta(1)S(82)beta, alpha(2) beta(1)Tm(82)beta and alpha(2) beta(1,82)Tm(82)beta, which were crosslinked between the amino groups of residues beta Val1 and beta Lys82 by 3,3’-stilbenedicarboxylic acid (S) or trimesic acid (Tm) while in the deoxy state, and saturated with carbon monoxide before crystallization. alpha(2) beta(1)S(82)beta, which has almost normal oxygen affinity, is completely in the R-state conformation; however, alpha(2) beta(1)Tm(82)beta and alpha(2) beta(1,82)Tm(82)beta, both of which have low oxygen affinity, have been prevented from completing their transition into the R state and display many features of a transitional intermediate, These haemoglobins therefore represent a snapshot of the nascent R state.