Dynamic sink strengths for the void, dislocation and thin foil surfaces are derived using a cellular model. These can be used in the chemical rate theory analysis of non-steady irradiation damage. In particular, appropriate dynamic sink strengths to model a steady state pulsing regime, such as is expected to prevail in a fusion reactor, are obtained. The transient nature of these sink strengths indicates the importance of the relative mobilities of the interstitials and vacancies and can, in fact, lead to the new phenomenon of a dynamic preference for vacancies, even for a sink that has a static preference for interstitials. The general dynamical features are found to depend also on both the irradiation mode and on the morphology of the particular sink. The present results indicate that for steady-state pulsing conditions it is usually acceptable to replace the dynamic sink strengths in rate theory calculations, by the corresponding steady state sink strengths appropriate to continuous irradiation. However, the dynamic sink strengths do appear to be required for the study of damage during the initial transient under continuous irradiation conditions.