Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M1−xFex(bpy)3](PF6)2 (M=Cd, Mn and Zn, x≈0.01, bpy=2,2'-bipyridine) under external pressure are presented. Below 50 K the high-spin→low-spin relaxation is an almost temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn1−xFex(bpy)3](PF6)2 and [Zn1−xFex(bpy)3](PF6)2 show a phase transition at ≈1.1 kbar, which increases the tunnelling rate by a factor of about 6.