An adsorbate monolayer composed of acetate (CH3COO-) and formate (HCOO-) was prepared on an atomically flat surface of titanium oxide as a two-dimensional analogue of substituted solid solution. When the mixed-monolayer was exposed to acetic acid (CH3COOH) vapor, an impinging acid molecule recognized a preadsorbed acetate among a lot of formates and replaced one of its formate neighbors. Straight chains of acetate were assembled in the monolayer as a result. A bimolecular intermediate state of the gas-adsorbate exchange was proposed to kinetically control which formate to be replaced.