Complementary molecular recognition in the gas phase onto self-assembled monolayers bearing thymine or adenine bases as terminal groups was studied by using a highly sensitive 63 MHz quartz-crystal microbalance. Association constants (K-a), binding and dissociation rate constants (k(1) and k(-1)), and binding amount in nanogram level (Delta m) could be obtained from time coarses of frequency decrease (mass increase) at various gaseous guest concentrations. Kinetic parameters were corrected to avoid the effect of different adsorption ability of guest molecules. Binding of 2-aminopyridine (an adenine model) onto a thymine monolayer showed the 50-100 times larger binding constant than that of a noncomplementary guest molecule, gamma-butyrolactam. On the other hand, an adenine monolayer preferentially bound the thymine model of gamma-butyrolactam over 2-aminopyridine, the adenine model. When a simple hydrocarbon monolayer of 1-decanethiol was used, all of these guest molecules were hardly bound. The selectivity between complementary and noncomplementary nucleobase pairs observed in the gas/thin film surface was comparable to the selectivity seen in organic solvents. The formation energy of hydrogen bonding obtained from K-a values in the gas/thin film surface was consistent with that calculated by molecular mechanics as in a vacuum. K-a values of the guest molecules were dependent on the nucleobase content in the mixed monolayer of nucleobase and alkyl chains.