The most common class of activation domains, the so-called acidic activators, has been proposed either to adopt an amphipathic alpha-helical structure or to exist as unstructured "acid blobs." However, genetic analysis of an acidic activation domain in the yeast GAL4 protein has suggested that the structure of the activation region is a beta sheet. To distinguish between these models, we conducted a biophysical analysis of peptides corresponding to the yeast GAL4 and GCN4 acidic activation domains. Circular dichroism spectroscopy shows that the peptides are not alpha helical, but that they can undergo a transition to a structure that is almost 100% beta sheet in character in slightly acidic solution. We also show that the artificial acidic activator AH has structural properties that are markedly different from the natural GAL4 and GCN4 domains and does not adopt a beta-rich structure at reduced pH.