In reactors having D2O or Be moderators, the photoneutrons produced by gamma rays from long-lived fission products give rise to transient effects which could introduce appreciable errors into various types of reactivity measurements. This paper presents digital computer calculations of these effects in D2O for criticality measurements, subcritical multiplication measurements, rod drops, and rising period measurements. It is found that in some cases appreciable errors are possible even after one hour waiting periods. Since the Be photoneutron data cannot be resolved into groups, calculations for Be could not be done. However, the nature of the effects that could be expected is discussed.