Adequate humidification of inhaling gases is important for the normal functioning of the respiratory tract. Humidification facilitates expectoration, inhibits the onset of respiratory complications. and prevents them from getting worse. It is especially important in the care of postoperative patients or patients who are under oxygen therapy. There are many types of humidifiers currently being used, but no reports have appeared in the literature on their efficiency. An improved photo tube dew-point hygrometer was used for measuring humidity, which was shown on an electronic recorder automatically and continuously. The temperatures of gases before and after humidification, of water in humidifiers, and of the environment air were measured with thermocouples simultaneusly. A number of experiments were done under varying conditions to evaluate how the efficiency of a bubble-type humidifier is influenced by factors such as: (1) the environment temperature, (2) temperature of gas before it enters the humidifier, (3) temperature of water in it, (4) depth of water for bubbling, (5) number, size and directions of holes from which oxygen-bubbles emanate, (6) flow of oxygen, (7) viscosity of water, etc. The following factors were found to increase the efficiency of humidifiers, experimentally and also theoretically, and were discussed from the standpoint of practical application. (1) Maintaining temperatures of the inlet flow of oxygen, which was the gas used in most cases, and of the water in the humidifier as high as possible. This is the most obvious factor seen from a clinical standpoint. Copper water containers are recommended instead of glass containers, making it possible to maintain the water temperature very close to the environment temperature. The environment should also be kept sufficiently warm. (2) Smaller size and increased number of holes (over ten holes with a diameter of less than 1mm.) can make good humidification, even with high flow. (3) Bubbling through increased water depth (over 10cm. of depth) is recommended. (4) Downward direction of holes is recommended. But, if there are many holes, they should be directed somewhat diverted to lateral so that bubbles radiate in all directions. (5) Increased viscosity of water is not useful for better humidification. Also, the effect of foaming and spraying over the water surface was obserbed.