BACKGROUND: During esophagectomy for esophageal cancer, meticulous attention is needed to prevent thermal injury to the vital organs, such as the recurrent laryngeal nerve (RLN) and tracheobronchus. In order to clarify the novel mechanism behind thermal injury induced by energy devices, we investigated the temperature of steam with the use of two different devices under wet and dry conditions. METHODS: An ultrasonic device (Sonicision™) and a vessel sealing device (Ligasure™) were studied. We evaluated the temperature at the tip of the devices and the steam when the devices were activated under different grasping ranges, under four different combinations of device and muscle, and under four different wet/dry conditions (dry-dry, dry-wet, wet-dry, and wet-wet). RESULTS: Although the maximum temperature of the devices was significantly higher with Sonicision™ than with Ligasure™, the maximum temperature of the steam was significantly higher with Ligasure™ than with Sonicision™ in almost all situations. At 1 mm away from Sonicision™, the critical temperature more than 60 °C was observed only when used with one-third grasping range under the wet-dry or the wet-wet conditions. In case of Ligasure™, high-temperature steam was observed when used with one-third grasping under the wet-dry or the wet-wet condition and two-third grasping under the dry-wet, the wet-dry, or the wet-wet condition. Under the wet condition, the emission of steam from the non-grasping part of Ligasure™ caused a spike in temperature that exceeded the critical temperature. CONCLUSION: We demonstrated that the use of energy devices under a wet condition generates steam from the non-grasping part of the devices. The temperatures of steam from Ligasure™ were significantly higher than that from Sonicision™. To prevent thermal injury to the vital organs, a very attentive and meticulous surgical technique is imperative considering the characteristics of each device.