A limited number of polar, nonhydroxylic organic liquids are suitable swelling media for chemical modification of wool. An effective medium for acylating-type internal modification should have low molecular weight, should lack active hydrogen atoms, be highly polar, swell wool fibers, have a high boiling point, and exert a catalytic effect. Rates of modification by phenyl isocyanate, myristoyl chloride, and dodecenylsuccinic anhydride in different media decrease in the order dimethyl sulfoxide > dimethylformamide > 1-methyl-2-pyrrolidinone > 4-butyrolactone. Nevertheless, equal maximum uptake of a given reagent in these varied media is evidence of internal chemical modification. Further examples of modifying reagents are phenyl isothiocyanate, benzyl chlorides, and α-haloketones. Supercontraction and single-fiber stress-strain data indicate crosslinking by bifunctional isocyanates and acid chlorides.