Copolymers for use in solubilizing substances which are insoluble in water, substances which are only sparingly soluble in water, and combinations thereof; such copolymers comprising: (a) 60 to 99% by weight of N-vinylcaprolactam; and (b) 1 to 40% by weight of at least one other monomer selected from the group consisting of: C.sub.8-C.sub.30-alkyl esters of monoethylenically unsaturated C.sub.3-C.sub.8-carboxylic acids; N--C.sub.8-30 alkyl-substituted amides and N,N--C.sub.8-30 dialkyl-substituted amides of acrylic acid, methacrylic acid, or combinations thereof; vinyl esters of aliphatic unbranched C.sub.8-C.sub.30-carboxylic acids; C.sub.8-C.sub.30-alkyl vinyl ethers; and mixtures thereof.