The steric opacity function (the dependence of reaction cross section upon mutual molecular orientation) for the endothermic Penning ionization channel of Kr(P-3)+(CH3)(3)CBr reaction was determined by using an oriented tert-butyl bromide molecular beam at 0.1 eV average collision energy. A remarkably large steric effect was observed in contrast with the Ar(P-3)+CH3Br reaction. We find that the ionization cross section is maximum at sideways and it becomes "zero" at the tert-butyl end. The Br end is found to be unfavorable as compared with sideways approaches. The stereo selectivity observed in the present work is discussed in terms of the electron exchange mechanism, in which the shielding effect by bulky nonreactive tert-butyl group as well as the smearing effect in impact parameter are taken into account. (C) 2003 American Institute of Physics.