Pyrene excimer emission in alkali-metal-exchanged zeolites is readily observed to grow in using an ultrafast streak camera with 3.3 ps time-resolution. Rise-time constants range from 7 to 14 ps depending on the composition of the zeolite. These results show that two pyrene molecules in doubly occupied cavities must move from their pre-excitation orientation into the face-to-face orientation required for the excimer.