When blending incompatible polymers in the melt, the resulting morphology is strongly dependent on the interfacial tension. One stage of the mixing process is now used to determine this interfacial tension: in the absence of an overall flow field, extended liquid threads in a liquid matrix exhibit sinusoidal disturbances which cause the threads to disintegrate into lines of droplets. From the growth rate of these disturbances, the interfacial tension between the thread phase and the matrix phase is calculated. For molten polymers, this so‐called ‘‘breaking thread method’’ is relatively fast and simple since it does not require density data for the two phases. Upon addition of a diblock copolymer to the thread phase, a considerable decrease in interfacial tension is measured.