Measurements have been made of ultrasonic absorption in carbon dioxide contained in narrow tubes of the order of 1 mm diameter. Frequencies used were between 40 and 120 kc/s. Results showed that over a considerable range of frequency/pressure on either side of the relaxation peak of CO₂ the total absorption was given by the sum of the Helmholtz-Kirchhoff tube absorption and the thermal relaxation absorption of the gas. The tube absorption, however, was found to be a few per cent higher than that calculated theoretically.