We have recently developed a solid phantom material having X-ray absorption and scattering properties closely simulating those of water. It is an opaque white homogeneous wax of density 0·99 gm./c.c. It is readily cut and machined and may be cast by pouring at about 120 deg. C. The composition by weight is: Paraffin wax 60·8 per cent. Polyethylene 30·4 per cent. Magnesium oxide 6·4 per cent. Titanium dioxide 2·4 per cent. Paraffin and polyethylene are indistinguishable as regards electron density and effective atomic number. The use of the specified hydrocarbon mixture ensures that when molten the wax pours easily and is yet viscous enough to keep the MgO-TiO2 filler in homogeneous suspension.