The solid state amorphisation reaction in a Ni50Mo50 alloy has been investigated by neutron diffraction and small angle scattering for a series of specimens mechanically alloyed for 2, 4, 8, 16 and 32 h. In the initial stages of the reaction, deformation of the parent crystalline phases occur. After 8 h of treatment, a change in the samples is observed and those treated for 16 and 32 h consist of an amorphous NiMo phase with some free molybdenum. Small angle scattering graphs of log I(Q) vs. log Q show a sequence of changes which correspond to the sequence of changes seen in the diffraction data, The graphs for the 2 and 4 h samples are linear over an extended range of scattering vector, Q. The slope of the graphs is about -3.4 which is indicative of a rough surface having fractal character between the transformed and untransformed regions of the samples