The results of a magnetochemical investigation of the solid solution KFe_xTi_1-2xNb_1+xO₅, isotypic with layered KTiNbO₅, are reported. The temperature dependence of the magnetic susceptibility points to a weak antiferromagnetism (T-fraktur sign ∼ -3 cm^-1); this is consistent with a fully ordered distribution of Fe³⁺ on one of the two sets of octahedra (Oc_I), as deduced from the XRD structure determination. Modeling the Fe-O-Fe exchange interactions in the diluted compositions (x ≤ 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.