The first detection and characterization of the interactions between the f-electronic systems in the dinuclear complexes of paramagnetic trivalent Tb, Dy, Ho, Er, Tm, and Yb ions with phthalocyanine ligands are presented. The molar magnetic susceptibilities, chi(m), were measured for PcLnPcLnPc* ([Ln, Ln]; Pc = dianion of phthalocyanine, Pc* = dianion of 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine) and PcLnPcYPc* ([Ln, Y]) in the range from 1.8 K to room temperature. The selective synthetic method previously reported for the heterodinuclear complex [Y, Ln] was used to prepare [Ln, Ln] and [Ln, Y] with a modification on the choice of starting materials. The f-f interaction contributions to the magnetic susceptibility are evaluated as Deltachi(m)T = chi(m)([Ln, Ln])T - chi(m)([Ln, Y])T - chi(m)([Y, Ln])T, where T refers to temperature on the kelvin scale. The homodinuclear complexes having f(8)-f(10)-systems, namely [Tb, Tb], [Dy, Dy], and [Ho, Ho], show positive Deltachi(m)T values in the 1.8-50 K range, indicating the existence of ferromagnetic interaction between the f-systems. The magnitude of the Deltachi(m) T increases in the descending order of the number of f-electrons. [Er, Er] gives negative Deltachi(m)T values in the 1.8-50 K range, showing the antiferromagnetic nature of the f-f interaction. [Tm, Tm] exhibits small and negative Deltachi(m)T values, which gradually decline in the negative direction as the temperature decreases in the range 13-50 K and sharply rise in the positive direction as the temperature falls from 10 to 1 8 K. [Yb, Yb] has extremely small Deltachi(m) T values, whose magnitude at 2 K is less than 1% of that of [Tb, Tb]. The ligand field parameters of the ground-state multiplets of the six [Ln, Y] complexes are determined by simultaneous fitting to both the magnetic susceptibility data and paramagnetic shifts of H-1 NMR. The theoretical analysis successfully converged by assuming that each ligand field parameter is a function of the number of f-electrons in each ton Using these parameters as well as the previously obtained corresponding parameters for the [Y, Ln] series, the interactions between the f-systems in [Ln, Ln] are investigated. All the characteristic observations above are satisfactorily reproduced with the assumption that the magnetic dipolar term is the sole source of the f-f interaction.