In the [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) spin-crossover system, the thermal spin transition is accompanied by a first order crystallographic phase transition (Tc↓ = 128 K and Tc↑ = 135 K) from RImage above Tc↓ to PImage at low temperatures (Wiehl L., Acta Cryst. B49, 289 (1993)). The high-symmetry phase can be super-cooled, in which case the spin transition is still complete and quite steep (T1/2 = 125 ± 2 K) but now without a hysteresis. The corresponding interaction constant Γ is 170 cm−1. In the diluted system [Zn1−xFex(ptz)6](BF4)2, X = 0.1, the spin transition is gradual with T1/2 = 95 ± 2 K. From the shift of T1/2 towards high temperatures with external pressure a value for ΔVHL0 of 26 Å3 molecule−1 is obtained. Pressures above 250 bar induce a crystallographic phase transition even in the diluted system, as a result of which the spin transition is discontinuous. The interplay between the thermal spin transition and the crystallographic phase transition in the neat and the diluted system is discussed consistently.