The global demand for reduced CO2 emission from power plants can be answered by coal gasification techniques. To develop integrated gasification combined cycles that incorporate hot syngas cleaning facilities, detailed knowledge of the thermophysical properties of coal ashes is imperative. Currently, the surface tension of liquid coal ash slags in a reducing environment was studied by means of the sessile drop method. Three different algorithms were employed to analyze the acquired drop images. the slag's under consideration. were obtained from black and brown coals as Well as from all experimental gasification reactor. Typically, a sharp surface tension decrease with temperature was found in the melting interval of the ashes. This was followed by it temperature range of smooth drop contours during which a slight rise of the surface tension could mostly be observed. Bubbles at the circumference of the drops started to appeal, when approaching the measurement temperature limit of 1550 degrees C, With regard to the temperature regime of uncorrugated drop profiles, coal ash slags exhibited surface tension values between 400 and 700 mN/m.