C-Si bond cleavage of p-trimethylsilylmethylacetophenone(1) occurred in a higher triplet excited state (T-n), giving mainly p-acetylbenzyl radical with the transient absorption in the region of 295-360 nm, with a quantum yield of 0.046 +/- 0.008 using the two-color two-laser photolysis techniques. In contrast, the C-Si bond cleavage of p-trimethylsilylmethylbenzophenone(2) was absent in the T-n state whose energy is larger than the C-Si bond cleavage energy. The results can explain existence of a bond cleavage crossing between potential surfaces of the T-n state and a dissociative state of the C-Si bond for 1, but not for 2. (c) 2005 Elsevier B.V. All rights reserved.