P. Sabine and E. Meyer have noted that for frequencies above 2,000 d.v. the rate of absorption of sound in a room increases as the relative humidity decreases. The writer has investigated and confirmed this effect during the past year and has obtained some quantitative data which have a bearing upon problems in architectural acoustics and sound signaling. Assuming that the intensity of a plane wave in air diminishes in accordance withe—mct where c is the velocity t the time and m the absorption coefficient in the air the reverberation formula becomes (in British units)t = .049V−S loge (1 − a) + 4mV.By making reverberation measurements, under different conditions of humidity, in two rooms having the same boundary material (painted concrete) but different mean free paths, it is possible to determine both m andaas functions of the frequency and humidity of the air. For frequencies below 512 d.v., m is negligible. It increases approximately with the square of the frequency. At 4,096 d.v., m increases from about .0015 at 70 per cent relative humdidity (21°C) to .0033 at 20 per cent relative humidity. At frequencies above 6,000 d.v., the absorption in the air in a room may be greater than the absorption by the boundaries. The absorptivity of painted concrete remains nearly constant above 512 d.v., and has a value of about .016.