The present invention provides a process and a device for regeneration of a nitrogen oxides storage catalyst arranged in the exhaust system of a diesel engine. The process comprises a first and a second regeneration strategy wherein the first regeneration strategy is applied when the exhaust gas temperature is above a threshold value and comprises changing the air/fuel ratio from a lean to a rich value during a first regeneration period and wherein the second regeneration strategy is applied when the exhaust gas temperature is below said threshold value and comprises switching the air/fuel-ratio between said lean and said rich air/fuel-ratio back and forth forming a sequence of rich/lean-pulses with a number of 2 to 10 rich/lean pulses during a second regeneration period.