The results for the Fischer-Tropsch reaction carried out in the slurry phase over an iron-based catalyst that deactivates linearly following a period of constant activity are reported. Hydrocarbon selectivity plots exhibit a single value of chain growth probability, alpha, at the shorter times on stream while two values of chain growth probability are observed at longer times on stream (during catalyst deactivation). This is shown to be consistent with a vapor-liquid equilibrium (VLE) phenomena associated with the slurry reactor. An unsteady-state VLE model is developed which successfully predicts the two values of chain growth probability (positive deviations) at longer times on stream even though single-a stoichiometry is assumed. Positive deviations of hydrocarbon selectivity plots have not been ascribed previously to VLE phenomena. Further, this behavior is shown for catalysts deactivating either linearly or nonlinearly and with or without an initial period of constant activity. Thus, double-a selectivity plots observed during catalyst deactivation may not be indicative of the intrinsic catalyst selectivity. This is shown to be due to the "flashing off" of lighter hydrocarbons previously accumulated in the liquid phase along with the hydrocarbons produced at the longer times on stream. The model predicts the hydrocarbon selectivity (single-alpha) at short times on stream, the values of the second chain growth probability at longer times on stream, and the increase in the extent of the positive deviations with time on stream. However, the model overpredicts the carbon number at which the break in the selectivity plots is observed; this is probably due to inadequate estimates of hydrocarbon saturation vapor pressures and the assumption of the ideal gas law.