Vocalizations between pairs of adult female “Gothic-arch” squirrel monkeys were tape-recorded, along with identification of the vocalizer and social context. The major purpose the study was to identify and describe acoustic characteristics of certain call types, and to attempt to correlate these characteristics with social context. The call type analyzed in this paper was the Twitter or Trill, an FM sound containing a limited set of repeated sound elements. Spectrographic analysis of these elements, or “syllables”, revealed that all Twitters were made up of a subset of the same 12 syllable types. Certain rules were identified which determine the sequential position of each syllable type in a Twitter. Syllable usage varied between individuals and with changes in social context.