In the gas phase the carboxyl group of an n-carboxylic acid molecule may adopt a number of different conformations. Upon adsorption on a solid surface, however; the carboxyl group usually assumes one of two possible configurations. Atomically resolved STM images show that the conformations of the carboxyl groups in 12-bromododecanoic acid and 2-bromohexadecanoic: acid molecules differ when adsorbed at the liquid (phenyloctane)-solid (graphite) interface.