The affinity constants (nk) between certain compounds and nucleic acids have been measured by equilibrium dialysis at 5[degree]C in buffered solutions containing 0.5[image] salt. In this formula n is the number of the binding sites and k is the average association constant of each site. The order of the increasing affinity constants of these compounds to the nucleic acids was found to be thymine adenine, caffeine < naphthalene < diethylstilbesterol < estradiol-[beta]-17 < testosterone < phenanthrene. The affinity constants of the nucleic acids in coil form was found to be much higher than those of nucleic acids in helical form. Desoxyribonucleic acid has a two order of magnitude higher affinity constant than ribonucleic acid. Nucleic acids tested were native and heat-denatured thymus DNA, helical poly A and coil poly A, poly U, ribosomal RNA, as well as yeast sRNA.