Abstract: Neutron activation is used to analyse the silicon and aluminium content of samples of material, such as bauxite ore and coal. The analysis can be performed on bulk samples, on material on a moving conveyor belt, or on the walls of a borehole. The method involves high energy neutron irradiation of the sample, measurement of the thermal neutron flux in the sample, and monitoring the gamma radiation from the sample at energies of (a) 1.78 MeV and (b) 0.844 MeV and/or 1.015 MeV. Such gamma radiation is produced on decay of (a) .sup.28 Al and (b) .sup.27 Mg isotopes, produced by the reactions .sup.28 Si(n,p).sup.28 Al and .sup.27 Al(n,p).sup.27 Mg. A single gamma ray detector is used. The analysis preferably utilizes equations which include terms to compensate for (a) the production of radiation at 1.78 MeV as a result of the production of the isotope .sup.28 Al from .sup.27 Al by the thermal neutron reaction .sup.27 Al(n,.gamma.).sup.28 Al, and (b) the Compton scattering of 1.