The present paper is focused on prevaporised ethanol flames in the transition from conventional combustion to the MILD combustion regime. Photographs and imaging of OH* chemiluminescence reveal a distinctive flame structure when ethanol carried by air burns in a 3% O-2 coflow (typical of MILD combustion), differing from that at higher oxygen levels. In comparison to flames carried by air in a 9% O-2 coflow, the spatial gradient and the peak in the OH* signal profile are significantly reduced in the 3% O-2 coflow, indicating a more uniform distribution of heat release and temperature. The use of N-2 as a carrier gas renders the OH* profile for the 6% O-2 coflow case similar to that of flames carried by air in the 3% O-2 coflow. The experimental results indicate a transition from conventional combustion to MILD combustion with the decrease of coflow O-2 level and/or the use of N-2 as a carrier gas. Calculations reveal that a substantial drop in the peak heat release rate and/or overall net heat release rate might contribute to the lack of luminosity of flames in the 3% O-2 coflow, suggesting a need for threshold values of these two in defining MILD combustion. A series of laminar flame calculations are performed to identify the MILD combustion regime based on the absence of a negative heat release region. The absence of a negative heat release region is found to be strain rate dependent at a given temperature and O-2 level of the oxidant stream. This is mainly a result of the enhanced transportation of O-2 across the reaction zone at a higher strain rate. At low strain rates, the negative heat release region is more likely to disappear in a 3% O-2 oxidant flow due to a combination of low flame temperature and high availability of O-2. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.