For the first time, formation of single crystals of ladderlike polyphenylsilsesquioxane (PPSQ) is found from three kinds of solution (benzene, toluene or p-xylene) at different temperatures (room temperature, 55, 65, 75, 80, 85 or 100 degrees C) for some time (from 10 min to 257 h). Crystallization conditions, such as temperature, time and solvent, have a great effect on number, size and shapes of PPSQ single crystals. The black crystals with shapes of square or rectangle have been prepared from benzene solution at 55 degrees C for not less than 51 h. The single crystals with the shape of a parallelogram, whose acute angles are 70 and 60 degrees, have also been obtained from benzene solution, however, at 65 or 75 degrees C, while those parallelogram-like single crystals with acute angles of 39 and 26 degrees have been obtained from toluene or p-xylene solution at 75 or 80 degrees C. When PPSQ was crystallized at room temperature (about 20 degrees C) or as high as 85 degrees C (greater than or equal to 85 degrees C), only small irregular crystals are observed. The largest parallelogram-like single crystal with the size of 200 x 75 mu m(2) was formed from toluene solution at 75 degrees C for 199 h. The size or number of PPSQ single crystals increases with an increase in the crystallization time. (C) 2000 Elsevier Science Ltd. All rights reserved.