L12-type Co3(Al,W) compound is a very important phase in Co-based high temperature materials. This study investigated the interdiffusions of Co-based Co-Al-W ternary alloys at 1 423, 1 473 and 1 523 K. Two types of diffusion couples were employed to estimate four diffusivity matrix coefficients ( , , and ). A modified ternary Boltzmann-Matano method was employed for the evaluation of interdiffusion coefficients. The estimated cross-interdiffusion coefficients ( and ) had opposite signs at 1 523 K which is not contradictory to but rather consistent with the Onsager's reciprocity theorem. The second derivative of the Gibbs free energy with respect to the Al concentration, ( ), may have a sign opposite to that of W concentration, (). Experimental results revealed that the effect of W on the Al diffusion flux is greater than that of Al on the diffusion flux of W by about one order of magnitude.KEY WORDS: interdiffusion coefficient; Co-based alloy; ternary alloy; Onsager's reciprocity theorem; Gibbs free energy.