The thermal and light induced spin transition in [Fe0.35Ni0.65(mtz)6](ClO4)(2) (mtz 1-methyl-1H-tetrazole) was studied by Fe-57 Mossbauer spectroscopy and magnetic susceptibility measurements. In addition to the spin transition of the iron(II) complexes the compound undergoes a structural phase transition. The high-temperature structure could be determined by X-ray crystallography of the isomorphous [Fe0.25Ni0.75(mtz)(6)](ClO4)(2) complex at room temperature. The X-ray structural analysis shows this complex to be rhombohedric, space group R (3) over bar, with a 10.865(2) Angstrom and c = 23.65(1) Angstrom with three molecules in the unit cell. The transition to the low-temperature structure occurs at similar to 60 K without changing the spin state of the molecules. By subsequent heating of the complex the high-temperature structure is reached again between ca. 170 and 200 K. The spin transition behavior is strongly influenced by the structural changes, and the observed spin transition curves are completely different for the high- and low-temperature phases. In the high-temperature structure a complete and gradual spin transition between 220 and 120 K (T-1/2((gamma HS) = 0.5) = 185 K) is detected; the high-spin (HS) state is represented by one HS doublet in the Mossbauer spectra. In the low-temperature structure a two-step transition curve is detected in the heating mode. About 36% of the molecules show a LS (low-spin) - HS transition between ca 50 and 75 g. Then the HS fraction stays constant up to 150 K. A further increase in the high-spin fraction is observed at temperatures above 150 K. In this structural phase the HS state is represented by two different HS doublets in the Mossbauer spectra. The formation of metastable HS states by making use of the LIESST effect is only possible in the low-temperature structure. By excitation of the LS molecules with green light, two different HS states are populated which show very different relaxation behavior. One HS state shows a relaxation to the LS state even at 10 K; the other HS state shows a very slow HS --> LS relaxation at 60 K (within days), leading to the HS fraction corresponding to the thermal equilibrium value.