A method for the measurement of the anisotropy constant of the antiferromagnet (AF) K-AF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature < T-B > of a CoFe/IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5 +/- 0.5)x10(6) erg/cc has been obtained at room temperature for a 4 nm thick IrMn layer. (C) 2007 American Institute of Physics.