Symmetric-top molecules (CF\sb3Br, CF\sb3Cl, CF\sb3H and CH\sb3Br) in a seeded supersonic nozzle beam are orientation selected by a hexapole electric field, and collide at a right angle with fast (3-40 eV) potassium atoms. The ionization reactions at two different molecular orientations are studied: $$\eqalign{&\rm K + CX\sb3 - Y \to K\sp+ + CX\sb3 + Y\sp-\qquad (Tails\ Orientation)\cr &\rm K + Y - CX\sb3 \to K\sp+ + CX\sb3 + Y\sp-\qquad (Heads\ Orientation)\cr}$$ We observed that collision ionization reactions are influenced greatly by molecular orientations. For CF\sb3Br, CH\sb3Br and CF\sb3Cl, the reactivities are greater with the heads orientation than that with the tails orientation, but for CF\sb3H, the H end is unreactive. The steric effects are more pronounced at the low energy end near the thresholds, and almost disappear at energies above 20 eV. Most importantly, we also found that the energy thresholds of these reactions are different for heads and tails orientations, indicating that the electron affinity of a molecule should be considered as an anisotropic parameter. Some features of the experimental results are explained by the Harpoon Electron Transfer model. Further theoretical and experimental studies are required for the fully understanding of the reaction dynamics.