The structure of DNA is the result of highly specific interactions between nucleotides (adenine and thymine, cytosine and guanine) based on hydrogen bonds and size complementarities. We performed direct measurements of the forces between adenosine and thymidine using a surface force apparatus [J. Chem. Soc. Faraday I 74, 975-1001 (1978)]. These measurements showed that without the size effect hydrogen bonds alone generate the specificity. Bond energies obtained in our experiments are consistent with estimates indirectly obtained through other methods. We have also observed an unexpected long-range nonspecific attraction.