The purpose of this study was to examine collaborative composing in high school string chamber music ensembles. Research questions included the following: (a) How do high school string instrumentalists in chamber music ensembles use verbal and musical forms of communication to collaboratively compose a piece of music? (b) How do selected variables (music performance and composing experience, quality of group collaboration, gender grouping, perceptions of the project) influence the process of composing collaboratively in a high school chamber music ensemble? and (c) What variables influence the quality of the resulting collaborative composition? High school string instrumentalists (N = 37) formed eight chamber music ensembles. Participants completed a presurvey regarding music performance and composing experiences, were videotaped while composing chamber music compositions, and completed a postsurvey regarding their project experiences. All groups spent a larger percentage of time engaged in task-directed musical communication than in verbal communication. Balance of collaboration had a strong relationship with composition quality. Mixed-gender groups had a stronger balance of collaboration than same-gender groups. Postsurvey data indicated that enjoyment of the project was found to have a strong relationship with composition quality.