A detailed in situ C-13 and H-1 NMR spectroscopic characterization of the following families of alkylperoxo complexes of titanium is presented: Ti(eta(2)-OOtBu)(n) (OiPr)(4-n), where n = 1-4; binuclear complexes [(iPrO)(3)Ti(mu-OiPr)(2)Ti(OiPr)(2)(eta(2)-OOtBu)] and [(eta(2)-OOtBu)(iPrO)(2)Ti(mu-OiPr)(2)Ti(OiPr)(2)(eta(2)-OOtBu)]; complexes with beta-diketonato ligands: Ti(LL)(2)(OEt)(eta(2)-OOtBu), Ti(LL)(2)(Oipr)(eta(2)-OOtBu), Ti(LL)(2) (eta(2)-OOtBu)(2), Ti(LL)(2)(OtBu)(eta(1)-OOtBu), where HLL = acetylacetone, dipivaloylmethane. These alkylperoxo complexes could not be isolated due to their instability and were studied in situ at low temperatures. Whereas the side-on (eta(2)) coordination mode of tert-butylperoxo ligand is generally preferable, the end-on (eta(1)) coordination caused by spatial hindrance from surrounding bulky ligands is found in two cases. The quantitative data on the reactivity of alkylperoxo complexes found towards sulfides and alkenes were obtained. The system TiO(acac)(2)/tBuOOH in C6H6 was reinvestigated using C-13 and H-1 NMR spectroscopy. The structure of the complex Ti(acac)(2){CH3C(O)(OOtBu)COO} actually formed in this system was elucidated. Four types of titanium(IV) alkylperoxo complexes were detected in the Sharpless-Katsuki catalytic system using C-13 NMR spectroscopy. (C) 2003 Elsevier Science B.V. All rights reserved.