Density and surface tension of slag are important properties for mineral processing, metal processing, slagging combustion and slagging gasification. In the case of slagging gasification, slag properties will impact the efficiency, reliability, maintenance cost and environmental performance of the process. There are several methods to determine the density and surface tension of slags. A commonly used technique is the sessile drop technique since it allows determination of both the density and surface tension, and can be performed at high temperatures in controlled gas atmospheres. This technique usually requires substrates that provide high contact angles with the samples for accurate results. To circumvent this issue, constrained sessile drops can be used. The limitations and accuracy of the constrained sessile drop technique for slag density and surface tension measurements were evaluated by a series of tests and phase equilibria predictions. Although molybdenum and alumina substrates provide lower contact angles than graphite substrates with the synthetic coal and petroleum coke slags tested, they interact less with the slags and alumina allows for measurements in both oxidizing and reducing conditions. Use of constraining 8 mm diameter molybdenum and alumina substrates resulted in greater precision than use of larger constraining and non-constraining substrates. The density of the synthetic coal slag studied is in the range of similar to 2.4 g/cm(3) for the temperature range of 1200-1600 degrees C, in both oxidizing and reducing conditions. Based on measurements with a molybdenum substrate, its surface tension is similar to 420 mJ/m(2) for the same temperature range. Measurements with alumina substrates, in both oxidizing and reducing conditions, yielded similar results above 1450 degrees C, but higher surface tension was observed at lower temperatures. Measurements with real coal slag provided similar results to measurements with synthetic coal slag. In the temperature range of 1530-1630 degrees C, the synthetic petroleum coke slag has a density of similar to 2.4 g/cm(3). Its surface tension is similar to 360 mJ/m(2) in oxidizing conditions and similar to 460 mJ/m(2) in reducing conditions. The surface tension difference is likely due to different phase equilibria with oxidizing and reducing environments, and may be impacted by the presence of solids in reducing conditions. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.