A series of four FeII coordination polymers [Fe(patrz)3]Cl2∙2H2O, [Fe(patrz)3]Cl2, [Fe(batrz)3]Cl2∙2H2O and [Fe(batrz)3]Cl2 was prepared with slight variations in the ligand framework (1,2,4-triazol-4-yl-propanoic acid (patrz) or 1,2,4-triazol-4-yl-butanoic acid (batrz)) and solvation (2H2O or dry). Dramatic influence of chain length and solvent has been evidenced on magnetic properties. Spin crossover properties were carefully studied by temperature dependent SQUID magnetometry and 57Fe Mössbauer spectroscopy. Thermodynamic parameters for [Fe(batrz)3]Cl2∙2H2O were determined by differential scanning calorimetry (DSC) allowing a simulation of the spin crossover behaviour and the quantification of the cooperativity.