The paper describes a novel method for the determination of ultraviolet (UV) light absorptivities for acid soluble lignin (ASL). It was found that the rehydrolysis, or reKlasonation, of Klason lignin continues to release a quantifiable amount of ASL. Thus, previously prepared Klason lignin was repeatedly subjected to reKlasonation and the mass loss after each stage of hydrolysis was quantified. The resulting hydrolysates were satisfactory for determining the absorptivity of ASL at 205 nm. The reKlasonation hydrolysates were found to be spectroscopically identical to hydrolysates prepared from the parent wood species.