The thermal spin transition in the diluted mixed crystal [Zn1 – xFex(6-mepy)3tren](PF6)2 (x = 0.00025, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for Δ H0HL and Δ S0HL of 1551(50) cm – 1 and 7.5(5) cm – 1/K and a molecular volume of reaction, Δ V0HL, of 22(2) Å3 result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)3tren](PF6)2 from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm – 1 and an interaction constant, Γ, of 109(15) cm – 1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1 – xFex(6-mepy)3tren](PF6)2 and the LS system [Zn1 – xFex(py)3tren](PF6)2 in the pressure range between 1 bar and 1 kbar are presented. Above ≈ 100 K the HS --> LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in [Zn1 – xFex(py)3tren](PF6)2 is more than three orders of magnitude larger than the one in [Zn1 – xFex(6-mepy)3tren](PF6)2. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.