Keilin and Hartree1 have described the sharpening and intensification of absorption spectra of haem compounds which occurred on making the observation at liquid air temperature. They observed that the intensification was due to light scattering by the ice crystals and could be duplicated by placing the cytochrome in a kaolin suspension. They also described a procedure for freezing the cytochrome in a glycerol–water mixture to a glass followed by devitrification by warming slightly and then refreezing the translucent material to give ten-fold intensification.