A new solubility test for wool is described. It is based on the extraction of wool in scoured fleece, top, or fabric form, by 5 M sodium iodide in aqueous ethanol (25% v/v) in the presence of tributylphosphine. The test is convenient and reproducible and is performed under mild conditions—neutral pH and room temperature. This solubility test is sensitive to modification of the wool and has been used to study the changes caused by scouring, hot water, mild alkali, and formaldehyde treatments.