Q-band electron paramagnetic resonance (EPR) data conclusively demonstrate that the iron and cobalt centers in the solid solution [Fe(bpp)(2)](0.97)[Co-(terpy)(2)](0.03)[BF4](2) (bpp = 2,6-dipyrazol-1-ylpyridine ) undergo allosteric spin-state switching during light-induced excited-spin-state trapping (LIESST) at 20 K and thermal relaxation around 80 K. EPR of [Cu(terpy)(2)](2+) and [Cu(bpp)(2)](2+) , doped into the same host lattice, also indicates expansion of the copper coordination sphere during LIESST excitation.