Composing is the act of creating something, often with a certain degree of skill and intentionality. Therefore, antonyms for "de-composing" would involve the opposite of creating or constructing something. Some potential antonyms might include "building," "constructing," "composing," "forming," or "erecting." Essentially, any word that implies the idea of creating or putting together could be seen as an antonym for de-composing. These words indicate the process of bringing something new into being rather than breaking it down or allowing it to decay. Overall, understanding antonyms for "de-composing" is useful in expanding one's vocabulary and understanding of language.