SCN − binding with guanidinated, acetamidinated, acetylated, carboxymethylated, β-dimethylaminopropionamidinated and cyanoethylated bovine plasma albumin has been studied. The binding at seven primary sites is not much affected in the modified proteins. Since the guanidinium groups are the only anionic binding groups unaffected by the modification reagents, this supports earlier evidence that the primary SCN − binding sites are located at arginyl residues. With respect to association at the secondary sites (presumably consisting predominantly of lysyl and imidazole groups) replacement of a hydrogen on the positively charged groups on the albumin by a bulkier radical is found to increase the binding constant of the site. Adducts which remove the positive charge on the binding site, on the other hand, appear to destroy the binding site.