Infrared absorption measurements of heavily carbon-doped AlAs have revealed a localized vibrational model at 630 cm-1 which appears as a Fano profile. Raman scattering measurements show a corresponding line at 634 cm-1: possible reasons for the difference in frequency are discussed. The line is assigned to CAs acceptors and a comparison is made with a two-parameter Keating cluster model. Additional weaker lines at 615 cm-1, 633 cm-1 and 645 cm-1 have been observed in absorption, and possible interpretations have been considered.