During ground testing and micro-g operation of Capillary Pumped Loops (CPLs), oscillations of the system pressure drop have been observed. In some cases, it is highly probable that they contributed to deprimes of the system when the magnitude of the pressure oscillations exceeded the capillary limit of the wick.A hydrodynamic stability theory was proposed in 1994 to explain the oscillatory behavior of the CPL systems. The theory has given insight to the cause of pressure oscillations in CPL systems and their effect on system operation. The theory indicates that the pressure oscillations are a function of the system design parameters and the operational conditions.One of the system parameters which affects the pressure oscillations is the wick spring constant of the porous wick structure in the evaporator. The wick spring constant is determined from porosity, pore size and permeability of the wick. During the development of the capillary heat transfer loops for the Earth Observing System Satellite (EOS) and the Capillary Pumped Loop 2 Flight Experiment (CAPL 2), several wicks were tested as part of the selection process for their CPL's capillary pumps. Each wick was tested in an identical loop using the same test procedures. It was observed that the different wicks produced different pressure oscillations under otherwise the same conditions.This paper will describe how the wick properties affect the pressure oscillations. A theoretical calculation of the wick spring constant which predicts the relative oscillatory trends will be presented for several different wicks. In addition these prediction will be compared with actual test data.