The affinity constants (nk) where n is the number of the binding sites and k is the average association constant of each site, between certain compounds and nucleic acids, have been measured by equilibrium dialysis at 5°C in buffered solutions containing 0.5 M salt. The order of the increasing affinity constants of these compounds to the nucleic acids was found to be: thymine ≪ adenine, caffeine < naphthalene < diethyl stilbesterol < estradiol-β-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 S-RNA.