Making use of the phenomenon of light-induced spin-crossover in the [Zn1−xFex(ptz)6](BF4)2 spin-crossover system, very high diffraction efficiencies η can be achieved in non-degenerate four-wave-mixing. In the mixed crystal with x=0.1 and at 76 K, i.e. at a temperature where the system is predominantly in the low-spin state, a value for η of 43% was obtained. This is attributed to a phase grating due to the large difference in metal---ligand bondlength between the low-spin ground state and the light-induced high-spin state. The rate constant for the decay of the laser-induced grating as a function of temperature is found to be exactly twice the one of the high-spin→low-spin relaxation, as expected for a dilute system in the absence of cooperative effects.