The behaviors of fires fueled by materials that produce different levels of soot (diesel and alcohol) were studied under hot and humid conditions with natural ventilation. Experimental tests were performed under different initial ambient temperatures (20 degrees C and 30 degrees C) and relative humidity levels (50 % and 90 %), and flame temperature, thermal radiation and mass loss rate were measured. A theoretical flame radiation model was established and verified. The fire development index (FDI) was defined as the ratio of the peak flame temperature, T-p, to the duration, t(p), during initial fire development stage, and the fire severity index (FSI) was defined as the product of the flame thermal radiation, R-av, and the duration, t(d), during fully developed stage. The results validated the reasonableness of the established theoretical model. During initial fire development stage, increasing the initial ambient temperature increased the FDI, while increasing the relative humidity decreased it. During fully developed stage, increasing the initial ambient temperature or humidity resulted in a decrease in the FSI under limited-fuel conditions, and the ambient temperature more significantly affected the pool fires fueled by alcohol than those that consumed diesel. Additionally, under conditions of high temperature and high humidity, both the indices were reduced. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.