Co-crystallization of [Fe(bpp)2][BF4]2 and [Co(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine) from nitromethane-diethyl ether yields homogeneous polycrystalline materials analysing as [FexCo1–x(bpp)2][BF4]2 (1.00 ⩽ x ⩽ 0.77). Thermal spin-crossover in these materials only involves the iron centers, and increasing the cobalt dopant concentration leads to a reduction in T½ and a loss of cooperativity. The materials exhibit the LIESST effect, with all three samples presenting the same T(LIESST) value. LIESST relaxation kinetics have a clear multistep character, which has not been detected before in samples derived from [Fe(bpp)2][BF4]2. Magnetic susceptibility and low-temperature crystallographic data are also presented for the pure precursor complex [Co(bpp)2][BF4]2.