Structural, magnetic and IR spectroscopic investigations were performed for two charge transfer salts of [Pt(mnt)₂]⁻ with 1-N-(4′-R-benzyl)pyridinium-d₅ (R = Cl or Br), which experience a magnetic phase transition around 274 K and 270 K, respectively. The crystals of the two salts are isostructural in both high-temperature (HT) phase, crystallized in monoclinic space group P2(1)/c, and low-temperature (LT) phases, crystallized in triclinic space group P. The anions and cations in the two salts form regular stacks in the HT phase, but dimerized ones in the LT phase. The two salts show one-dimensional S = 1/2 Heisenberg antiferromagnetic uniform chain magnetic nature in the HT phase, but spin gap behavior in the LT phase. By comparison of the analogues of [Pt(mnt)₂]⁻ with 1-N-(4′-R-benzyl)pyridinium (R = Cl or Br), the different isotopic effects are discussed for the two salts.