In [Fe(mtz)6](BF4)2 (mtz = 1-methyltetrazole), a spin transition of 50% of the Fe(II) ions corresponding to one crystallographic site occurs at 74 K. Thermal trapping in the HS state of the spin transiting site is achieved quantitatively. Relaxation back to the LS state occurs in the range 45-70 K. Non-first-order kinetics are reproduced using a phenomenological model including a self-acceleration parameter α = 1.8. The Arrhenius plot shows that a thermally activated process is dominant in the range of temperatures studied, with an activation energy Ea = 6.02 kJ mol-1. The speed of cooling and the applied field are shown to be of small influence on the kinetics. A new member of the propyltetrazole family, [Fe(ptz)6](CF3SO3)2·2ptz (ptz = 1-propyltetrazole), has been synthesized, which has two non-coordinated ptz molecules in the lattice. Its steep transition at 178 K is characterized by magnetic measurements, Mössbauer spectroscopy, reflectivity and calorimetry. The LIESST effect is shown to be present at 10 K, but only on the surface and with an extremely short lifetime. © 2001 Elsevier Science Ltd.