The results of a magnetochemical investigation of the solid solution KFexTi1-2xNb1+xO5, isotypic with layered KTiNbO5, are reported. The temperature dependence of the magnetic susceptibility points to a weak antiferromagnetism (J approximate to -3 cm(-1))approximate to -3 cm(-1)); this is consistent with a fully ordered distribution of Fe3+ on one of the two sets of octahedra (Oc,), as deduced from the XRD structure determination. Modeling the Fe-O-Fe exchange interactions in the diluted compositions (x less than or equal to 0.075), from the concentration and temperature dependence of the magnetic susceptibility, gives evidence of Fe3+ aggregation phenomena. For the lowest concentrations, such clustering can be described in terms of only dimers. The as-calculated values of the amount of dimers, for example 36% for x = 0.045, are found to be much larger than that corresponding to a random distribution of Fe3+ on the Oc(I) site.