Singlet molecular oxygen O 2( 1Δ g) arising from the thermodissociation of the endoperoxide of 3,3′-(1,4-naphthylidene) dipropionate (NDPO 2) was used to assess the quenching ability of various thiols and related compounds in sodium phosphate buffer in D 2O at 37 °C. The overall quenching ability decreases in the sequence ergothioneine, methionine, cysteine, β, β-dimethyl cysteine (penicillamine), mercaptopropionylglycine, mesna, glutathione (GSH), dithiothreitol, N-acetyl cysteine and captopril. Cystine, glutathione disulphide, dimesna, methionine sulphone and methionine sulphoxide have no quenching effect. Comparison of the rate constants for physical ( k q) with chemical ( k r) quenching by thiols indicates that chemical reactivity accounts fully for their ability to quench O 2( 1Δ g), and pD dependence indicates that the thiolate anion reacts with O 2( 1Δ g). Loss of thiol groups, as exemplified by GSH, is not affected by the free radical scavengers superoxide dismutase and mannitol. However, sodium azide, a scavenger of O 2( 1Δ g), completely prevents NDPO 2-induced thiol depletion. Depletion of GSH by NDPO 2 is accompanied by the formation of its disulphide, sulphinate, sulphonate, sulphoxide and other products.