A heat-sensitive recording unit is formed by applying a chromogenic reactant of the methine type and an oxidising organic compound supported on a base support. A regulator for the sensitivity of colour generation can also be applied to the base support. It is also possible to add a quaternary ammonium salt, a tertiary alkanol amine or a water-soluble ion scavenger, in order to avoid undesired colour generation by the chromogenic reactant of the methine type. The ammonium salt, the amine or the iron scavenger can be added during the formation of the chromogenic reactant of the methine type through an aqueous suspension, in order to apply the chromogenic reactant of the methine type to the base support.
