The transient behavior of the flux from a valved effusion cell containing red phosphorus is shown to result from a low elemental sublimation coefficient (&agr;=1.3×10−7) and not from the presence of white phosphorus in the sublimator region. The criteria for generating white phosphorus in the sublimator are given. The conversion from tetramers to dimers is modeled as a function of temperature and pressure. At higher pressures, higher temperatures are needed to convert P4to P2. Experimental results are shown to be in qualitative agreement with results from thermodynamic equilibrium arguments. ©1995 American Institute of Physics.