A method of preparing Group XIII selenide nanoparticles comprises reacting a Group XIII ion source with a selenol compound. The nanoparticles have an MxSey semiconductor core (where M is In or Ga) and an organic capping ligand attached to the core via a carbon-selenium bond. The selenol provides a source of selenium for incorporation into the semiconductor core and also provides the organic capping ligand. The nanoparticles are particularly suitable for solution-based methods of preparing semiconductor films.