This paper aims to study the guidelines followed by eight international auxiliary languages (IAL), a paradigmatic subgroup of so-called languages a posteriori, in the formation of new words. To accomplish this porpouse, we will explain how the designers of these languages –Volapük, Pasilingua, Esperanto, Lengua Católica, Langue Bleue, Idiom Neutral, Novial and Interlingua– solve the need to provide them with procedures that improve their vocabularies. Moreover, we will describe those mechanisms and its similarity to those of natural languages, and we will analyze the way in which these artificial languages solve the semantic and grammatical problems which affect to word formation in natural languages (allomorphy, polysemy, synonymy...).