Spin-crossover (SCO) complex [Fe(tpa)(NCS)(2)] (tpa = tri(2-pyridylmethyl)amine) crystallized in two solvate forms, yellow [Fe(tpa)(NCS)(2)]center dot X [Fe:X = 1:1; X = n-PrOH (complex is named as n-PrOH), i-PrOH (i-PrOH), CH2Cl2 (CH2Cl2), CHCl3 (CHCl3), MeCN (MeCN)] and red [Fe(tpa)(NCS)(2)](2)center dot Y [Fe:Y = 2:1; Y = MeOH (MeOH), EtOH (EtOH)], respectively. Between the two forms, interesting solvent-vapor induced in situ reversible and irreversible guest molecule exchanges, [Fe(tpa)(NCS)(2)]center dot X reversible arrow [Fe(tpa)(NCS)(2)](2)center dot Y, occurred in the solid state followed by dramatic color changes as well as distinct structural and SCO behavior transformations. Comprehensive studies on structures and SCO behaviors associating guest exchanges have been conducted by X-ray single-crystal diffraction, PXRD, IR, elemental analysis, and magnetic measurements, respectively. This discrete molecular system shows unique solvent-dependent S CO behavior related to the nature of solvent molecules; the distinct color changes during guest exchange originate from the alternations of electronic states of the guestsensitive Fe-II centers, providing an effective route to fine-tune and optimize materials' properties by systematic structural perturbation, or serving for detection of toxic gases, such as CH2Cl2 and CHCl3.