A series of spin transition (ST) iron(II) compounds of the type [FeIIL2](X)2·{S}2 (where L is 4′-(4‴-cyanophenyl)-1,2′:6′1″-bispyrazolylpyridine, X = ClO4− or BF4−, and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A·{S}2) crystals of the composition [FeII(L)2](ClO4)2·{S}2, and two polymorphic compounds of the stoichiometry [FeII(L)2](BF4)2·{S}2 as red coffin (B·{S}2) and orange block (C·{S}2) crystals. The Fe–N bond distances of A·{S}2 (from 1.921(9) to 1.992(3) Å; at 150 K), B·{S}2 (from 1.943(2) to 2.017(2) Å; at 180 K) and C·{S}2 (from 1.883(3) to 1.962(3) Å; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe–N distances at 180 K for the two polymorphs B·{S}2and C·{S}2 are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS) ↔ orange (HS)) for A·{S}2 and B·{S}2 and ca. 3.6% decrease in the unit cell volume of A·{S}2 from 4403 Å3 at 300 K to 4278 Å3 at 150 K. The temperature dependent magnetic susceptibilities of A·{S}2 and B·{S}2 demonstrate systematic increase of the spin transition temperatures (T1/2) and continuous decreases of the hysteresis loop width (ΔT1/2) upon slow lattice-solvent exclusion.