Abstract The E. S. R. spectra of π-(C5H5)2 TiCl2 AlR3 complexes and some related ones are studied. From the hyperfine patterns of these complexes in toluene solution and of the complexes dissolved in excess A1R3, we propose that the unpaired electron is localized in the neighborhood of the Al27 nucleus, and the unpaired electron cloud may extend to the a-hydrogen positions. Solvents mainly play the role of dispersing the complexes so as to reduce the spin spin interactions between them. Due to the hyperfine interactions of H1 and Al27 nuclei, the spectra show complex hyperfine patterns. However, the interaction ability of H1 nucleus is stronger than that of Al27 nucleus. Therefore when the solution is very concentrated, the spectra show only a single line due to the spin-spin interaction between the complexes. As the concentration decreases, the H1 nucleus first makes their contribution to the hyperfine interaction showing a quadruplet or six line spectra. On further diluting the solution, both H1 and Al27 nuclei contribute to the interaction, but their ability of coupling with the unpaired electron is non-equivalent. Only when the solution concentration becomes extremely dilute, the hyperfine interation of H1 and that of Al27 become equivalent and therefore the spectra with eight lines of relative imtensiay 1:3:4:4:4:4:3:1 could be exp-lained. The hydrogen bridge structure suggested by Зефирова and her collabceabors seems to be lacking of sufficient experimental ground.