We studied the interfacial tension between coexisting colloid-rich and polymer-rich fluid phases in a phase-separated colloid-polymer mixture. First, the location of the fluid-fluid binodal and the tie lines between the coexisting phases were determined using a recently proposed method that only requires the volumes of the coexisting phases as input. The spinning drop method was used to determine the very low interfacial tension between the coexisting phases. We also used the dynamics of the breakup process of the droplet of one phase suspended in the other to determine the interfacial tension.