Several absorption bands of C2D2 observed at 4.6, 3.4, and 3.1 μ have been measured and analyzed by employing a high resolution infrared spectrograph. Molecular constants for the levels involved in the transitions have been evaluated. A precise value for q5 = 0.0032575cm-1 has been determined from the structure belonging to the Q, P, and R branches of the ν1 - ν51 band. © 1968.