In this work we present results obtained by the first-order reversal-curve (FORC) method for finding the effect of pressure on a spin transition. The FORC data were recorded by diffuse reflectivity measurements for the spin-transition compounds [Fe(btr)2(NCS)2]•H2O and [Fe0.6Zn0.4(btr)2(NCS)2]•H 2O (btr = 4,4′-bis-1,2,4-triazole) under constant pressure in the range 1-1600 bars. The joint distributions in coercivity-bias coordinates, obtained by the FORC method, were expressed in energy gap Δ and interaction parameter J coordinates for a complete discussion of the pressure effect in terms of physical parameters, including their average values, distribution widths, and Δ-J correlation parameter. Pressure increases both Δ and J, as expected. Pressure has a negligible effect on the distribution widths but sizably decreases the correlation parameter value of the diluted system, an unexpected feature which suggests that the diluted system has multiple-domain behavior with pressure-dependent domain size. This deduction is supported by inspection of the samples using optical microscopy at room temperature. Simulations of the metal distribution in a two-dimensional lattice are performed in order to estimate the pressure-induced change in domain size. © 2011 American Physical Society.