Human communication compresses a massive amount of conceptual structure into conventionalized speech patterns. How do these patterns emerge? Experimental semiotics offers new empirical techniques to address such questions (Fay et al., 2008; Galantucci, 2005; Garrod et al., 2007; Kirby et al., 2008). We devised an experiment to observe the emergence of communicative conventions. The study explores the development of vocal communication systems through an iterative Vocal Charades game. The game is played by two players each given a stack of 12 cards. Written on each card is a word from six antonymic pairs (rough/smooth, bad/good, etc.). Over ten rounds, players took turns “vocalizing” the meaning of their words without using language or gestures. Analyses reveal how sounds conventionalize, leading to stereotyped forms and improved guessing. The conventional forms that develop are predicted by iconic correspondences with meaning, but also exhibit more arbitrary features that distinguish semantically similar words.