The native affinity of the nucleobases of DNA to form pairs is investigated by studying the gas phase behavior of clusters of free unsubstituted nucleobases and mononucleosides. The formation of neutral bimolecular clusters of these compounds in the gas phase has been studied by IR laser desorption of the neutral molecules into a supersonic beam expansion. Complementary nucleobase pairs as found in DNA, these being adenine-thymine and cytosine-guanine pairs, have been found to be formed in preference to noncomplementary base pairs. "Pseudo association constants" for the formation of the dimers of free nucleobases and nucleosides in the gas phase are calculated from the experimental results. A strong influence due to side groups affecting the dimer formation of the nucleobases is shown.