Direct measurement of surface forces has been employed for the systematic determination of interaction forces between nucleic acid base monolayers at various pH values from 5.6 to 9.3. Measured forces have been classified as (1) forces at a distance D > 20 nm, which are attractive or repulsive depending on the hydrophobicity and the charge of the surfaces and which can be accounted for in terms of the sum of "double-layer forces" and so-called "very long ranged hydrophobic attraction"; which are always attractive; and (3) pull-off (adhesive) forces from contact, which reflect characteristics of molecular contact between monolayers of nucleic acid bases. The forces between complementary pairs (adenine-thymine) are found, for the first time, to be always attractive and stable independently of separation distances as well as pH’s studied. On the other hand, interactions between noncomplementary pairs (thymine-thymine, adenine-adenine) are found to be regulated by the surface charge; thus, notably the long-ranged forces change from attractive to repulsive at pH’s where the nucleobases start to dissociate.