The iron mixed-valence complex (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe-II and Fe-III occurs reversibly at similar to 120 K, in addition to the ferromagnetic phase transition at T-C = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C3H7)(4)N[(Fe1-xZnxFeIII)-Zn-II-Fe-II(dto)(3)] (dto = C2O2S2; x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and Fe-57 Mossbauer spectroscopy. With increasing Zn-II concentration (x), the CTPT is gradually suppressed and disappears at x approximate to 0.13. On the other hand, the ferromagnetic transition temperature (T-C) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn-II ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn-II concentration. This anomalous dependence of T-C on Zn-II concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.