Cation substitution effects on structural and optical properties are investigated for films of Prussian-blue-type cyanides, (Na1-xKx)(0.35)Co[Fe(CN)(6)](0.74)zH(2)O(0.07 <= x <= 0.85). Substitution of K+ for Na+ induces a phase transition from high-spin (HS) phase with the electronic configuration of Co2+(t(2g)(5)e(g)(2))-Fe3+(t(2g)(5)) to low-spin (LS) phase with the electronic configuration of Co3+(t(2g)(6))-Fe2+(t(2g)(6)). At around room temperature, the HS phase (LS phase) is observed in the region of x <= 0.2(x >= 0.8) while two phases coexist in the intermediate-x region (0.2 <= x <= 0.8). We interpreted the two-phase region in terms of intrinsic phase separation into small-x (Na+-rich) and large-x (K+-rich) regions because the cations can migrate within the crystal to minimize the total free energy. We further confirmed that absorption spectra in the infrared, visible and Co/Fe K edge regions are quantitatively reproduced by the phase-separation model.