Chemical reactions in shock waves can be strongly affected by minute impurity concentrations. Thus it is not adequate to take into account the additional impurity electron production in relaxation studies simply by global adjustment of the atom—atom excitation cross-section constant to the measured electron density. A definite improvement, however, can only be achieved if the ionization relaxation model is extended to include all relevant impurity atom reactions. Consequently we treated the real test gas as a mixture of krypton and impurity carbon atoms. For the carbon model it is important to take the lower real excitation levels into consideration. Carrying out a sensitivity analysis we were able to reduce the number of reactions substantially. A comparison with experimental electron density profiles yielded 3. 0 × 10–-6 m2/J for the Kr—Kr excitation cross-section constant as well as values for the C—Kr constants. For a temperature of about 8000 K and an impurity concentration of about 40 p. p. m. it is shown that the impurity reactions dominate the electron production in the initial relaxation zone. This effect causes a pronounced decrease of the relaxation time with increasing concentration. By comparing computational results of the Kr—C model with those of the simplistic pure Kr model it is possible to explain the dependence of the Kr-Kr excitation cross-section constant on the impurity concentration and the plasma temperature. © 1991, Cambridge University Press. All rights reserved.