An apparatus and method for humidification of inspired gases, wherein the present invention utilizes moisture from condensed expiratory gases deposited within the outer expiratory tube of a conventional unilimb breathing circuit to humidify oxygen gas (or any other inspiratory gas) for subsequent patient inhalation, and wherein the oxygen gas may be directed through the outer expiratory tube via a novel reverse flow adapter coupled to an oxygen gas source. The present invention preferably functions to effectively eliminate prior art methods of oxygen gas humidification that depend upon the wasteful utilization of bottles of sterile water, corrugated tubing, nebulizer adapters and excess consumption of oxygen gas; thus, effectuating a cost savings for the patient and contributing to overall environmental conservation efforts.