Heat transfer under nucleating bubbles were directly measured using a microheater array with 100 mum resolution with low and high subcooled bulk liquid along with images from below and from the side. The individual bubble departure diameter and energy transfer were larger with low subcooling but the departure frequency increased at high subcooling, resulting in higher overall heat transfer. The bubble growth for both subcoolings was primarily due to energy transfer from the superheated liquid layer-relatively little was due to wall heat transfer during the bubble growth process. Oscillating bubbles and sliding bubbles were also observed in highly subcooled boiling. Transient conduction and/or microconvection was the dominant heat transfer mechanism in the above cases. A description of the experimental setup, the results, and implications for modeling boiling heat transfer are presented. A transient conduction model is developed and compared with the experimental data. (C) 2004 Elsevier Ltd. All rights reserved.