Three denatured states of bovine pancreatic trypsin inhibitor have been characterized, using two chemically synthesized analogues designed for study of folding intermediates. One analogue, [14-38]Abu, retains only the 14-38 disulphide. At pH 4.5-6 and 1-7 °C, [14-38]Abu is a highly ordered β-sheet molten globule; it has the circular dichroism (CD), ANS-binding and folding kinetics of a molten globule; is partially folded by NMR analysis; and undergoes cooperative thermal denaturation. At low temperature [14-38]Abu also forms an acid state at pH 1.5, as well as a denatured state at pH 2.5. A second BPTI analogue with all three disulphide bridges eliminated, [R]Abu, lacks detectable secondary and tertiary structure but has stable hydrophobic surfaces and is collapsed. We term this species a ‘molten coil’.