Precursor - A material consisting of fuels and oxidisers that are mixed and where the material is intended to be used exclusively in the production of an explosive. In order for such substances to be categorised as a precursor, evidence shall be produced demonstrating that the substance cannot cook-off leading to a mass violent reaction.

An example of an explosives precursor is ungassed emulsion.

As precursors can be shown not to detonate they are generally classed as 5.1 Oxidising Agents and not as a 1.1D explosive.

To change a 5.1 precursor into an 1.1D explosive a sensitiser must be added. This is generally in the form of nitrogen bubbles (gassing) which will reduce the density to below 1.3 g/cc.