Nitrosation of Bovine Serum Albumin (BSA) by nitrous acid occurs in two stages: a rapid step, which is consistent with an S-nitrosation reaction of a free cysteine unit, followed by a slower reaction which is consistent with an N-nitrosation reaction of a tryptophan residue. The fast reaction is catalysed by chloride and bromide ions (as is the reaction of free cysteine), whereas the slower reaction is not halide ion catalysed (like the reaction of tryptophan). Kinetic results were obtained for both reactions. The derived rate constants for the first stage for the reaction of ClNO and BrNO are reasonably close to the reported values for the reactions of cysteine. The second stage is a reversible process and we can estimate from measured infinity values and also from the variation in the measured rate constant for reactions at different [HNO₂], values for the equilibrium constant of 3500 ± 200 and 2600 ± 200 dm³ mol⁻¹ which compare reasonably with the reported value for the reaction of tryptophan of 850 dm³ mol⁻¹. The pK_a values of the cysteine residue in both BSA and Human Serum Albumin (HSA) were determined from rate measurements of the reactions with dipyridin-4-yl disulfide (4-aldrithiol), and yielded values of 8.32 and 8.18 respectively, which are close to the accepted value of 8.4 for cysteine itself, and which are substantially higher than the much quoted literature values. The S-nitroso derivatives of both BSA and HSA generated in solution (at ∼1 × 10⁻⁴ mol dm⁻³) showed very little sign of decomposition at pH 7.4 when measured spectrophotometrically, even in the presence of added Cu²⁺. There was, however, clear evidence of rapid decomposition at much lower reactant concentration (∼1 × 10⁻⁶ mol dm⁻³) yielding nitric oxide in increasing amounts and rates, as Cu²⁺ is added. These results are discussed in terms of the complexation of the Cu²⁺ catalyst at different albumin derivative concentrations. Transnitrosation between the S-nitroso derivatives of both BSA and HSA and excess cysteine occurred very readily. These experiments were carried out in the presence of added Cu²⁺, and the decomposition of the resulting S-nitrosocysteine (S-NOCys) was monitored. The results support the suggestion that S-nitroso proteins can, in theory, act as a reservoir for NO which can readily be made available, either by the Cu²⁺-catalysed reaction, or more readily by a direct NO⁺ transfer to a low molecular weight thiol, such as cysteine, which is much more prone to yield NO rapidly and quantitatively by the Cu²⁺-catalysed pathway. The S-nitroso derivative of BSA also underwent a relatively slow decomposition reaction initiated by cysteine, even in the presence of EDTA. This is discussed in terms of a reaction yielding nitrite anion which is not derived from NO.