The droplet combustion characteristics of conventional heating oil, UCO biodiesel and their mixtures (10 and 20% biodiesel by volume) were examined in a free-falling droplet combustion facility designed to approach the typical temperature and oxygen conditions around individual droplets in real boiler flames. Moreover, and given the importance of avoiding effects due to gravity or convection in this type of experiments, the quite spherical soot shells recorded (infrequent event in free-falling droplet combustion studies) reveal a configuration close to 1-D, even though tests were performed at normal gravity. The droplet combustion characteristics were evaluated at four different oxygen levels (0,3,5 and 10% O-2), and comprise droplet size evolution, burning rates and sizing of the occurring soot shells and envelope flames. Results show indistinguishable behaviors between heating oil and their mixtures, whereas biodiesel displays longer initial heating transients and higher quasi-steady burning rates. Even though the UCO biodiesel studied is highly unsaturated, no signs of polymerization were observed for any of the tests. In addition to these combustion characteristics, an aspirating soot probe was used to quantitatively evaluate the propensity to soot of each fuel at different oxygen levels. The amount of collected soot was found to significantly decrease with biodiesel addition, even though similar results were obtained for both mixtures. The practically identical droplet combustion characteristics between heating oil and its mixtures with biodiesel, in addition to the lower propensity to soot of the latter, support the feasibility of using such blends as drop-in replacement for conventional heating oil. The experimental results obtained are thought to constitute a valuable database for the development and validation of modeling tools for the prediction of droplet evaporation and combustion of practical fuels in realistic conditions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.