The molecular structure of polyaluminocarbosilane (PACS) derived from polycarbosilane (PCS) was investigated by (1)H, (29)Si, (27)Al NMR and elemental analyses. Quantitative methods based on experimental data were established to determine the exact amounts of O-containing groups, namely Si-O-Al and Si-O-Si, and the relative numbers of linear and cyclic Si-C segments in PACS. The HSiC(2)O groups and the SiH/SiH(2) in linear Si-C segments, single and condensed six-member Si-C rings were quantitatively distinguished. The results showed that PACS molecules consisted predominantly of single six-member Si-C rings. The introduction of Al into PCS led to H consumption through which Al was closely bonded to O by breaking Si-H or Si-CH(3) to form Si-O-Al cross-linking bonds and AlO(x) (x = 4-6) groups in PACS. The AlO(x) (x = 4-6) groups acted as linkage centers to bridge smaller PCS molecules through which the molecular weight and its distribution coefficient of PACS were both apparently increased. (C) 2011 Elsevier Ltd. All rights reserved.