Solvolyses of isobutyl chloroformate (4) in 43 binary solvent mixtures including highly aqueous media, water, D 2 O, CH 3 OD, 2,2,2-trifluoroethanol (TFE) as well as aqueous 1,1,1,3,3,3-hexafluoro-isopropanol (HFIP) solvents were performed at 45 solvolyses of 4 are found to be consistent with the proposed mechanism (Ad E ). The variety of solvent systems was extended to comprise highly ionizing power solvent media (Y Cl > 2.7 excepted for aqueous fluorinated solvents and pure TFE solvent) to investigate whether a mechanistic change occurs as solvent compositions are varied. However, in case of 18-solvent ranges having aqueous fluorinated solvent systems (TFE-H 2 O and HFIP-H 2 O) and/or having Y Cl > 2.7 solvent systems, the solvent effect on reactivity for those of 4 are evaluated by the multiple regression analysis as competition with S N 2 -type mechanism. And in pure TFE and 97 w/w % HFIP solvents with high Y Cl and weak N T , these solvolyses are understood as reactions which proceed through an ionization (S N 1) pathway.