Exhaust gases are dosed with reductant solution (R) before passing through a catalyst (6). The addition is calculated from the requirement. Reductant solution density is used in determining the volumetric dosage rate. Preferred features: Reductant solution temperature is measured, allowing the density to be calculated, which is used to determine the dosage volume. A characteristic curve determines dosage and gives the temperature-density relationship. The reductant is tempered and the density is adjusted. An NTC (negative temperature coefficient) heating element carries out tempering. Reductant concentration is used in determining the quantity to be dosed. An Independent claim is included for corresponding equipment which includes a reductant reservoir (8) and unit dosing reductant into the exhaust gases (A). A controller (18-26) determines reductant volume required, from its density.
