The problem discussed is the calculation of the time it takes for impurity atoms and ions to become ionized in the hydrogen plasma produced in pulsed thermonuclear machines. Classical ionization cross sections are used to calculate ionization rates and the effect of radiative recombination is included. The importance of taking into account the time dependence of the state of ionization is illustrated by an example where the energy radiated by an impurity of carbon is estimated. The time for impurity ions to reach the steady state is compared with the duration of the discharge for a number of operating thermonuclear machines and it is found that the latter times are too short for the steady state to be established. The assumption of the steady state is shown to be acceptable for a power producing D-D reactor.