The results of dynamic analysis of the effect of pressure variations during helium boiloff experiments are presented. A general solution of the diffusion equation with a timedependent boundary condition is employed to describe the dynamic response of the liquid helium system under variable pressure conditions, and a solution is obtained for the special case when the system is subjected to a step change in pressure. The calculated temperature response of the liquid indicates that most of the experiments were not likely to have reached equilibrium as a result of the low thermal diffusivity of liquid helium. The initial rate of evaporation or condensation is large, and the rate decreases sharply with time. A method is proposed to account for the transient effect that is observed during calculation of the heat loss rate from a helium boil-off experiment. By assuming that there is no mixing at all, the present analysis provides an estimate of the upper (condensation) or lower (evaporation) bound of the heat Iossrate as a result of a pressure increase or decrease in the system. A previously reported equilibrium analysis is expected to apply to situations where complete mixing occurred in the bulk liquid and provides the opposite limits.