Like water, alcohols are amphoteric. They can serve either as a proton donor or a proton receiver. In the presence of a strong acid, the basicity of a secondary or tertiary alcohol can lead to carbocation formation. First, the alkyloxonium ion is formed (the conjugate acid of the alcohol) which is followed by the water acting as a leaving group. The result is the carbocation which can serve as the substrate for addition to nucleophile, completing the SN1 pathway. Because the SN1 mechanism in the reaction of alcohols with HX involves a carbocation intermediate, rearrangement to form more stable carbocation can occur prior to capture by halide.

SN2 can also occur upon alkyloxonium cations. Whether the reaction will occur by either the SN1 or SN2 mechanism depends mainly on whether the alcohol is primary, secondary, or tertiary. Whether the leaving group is the halide ion discussed in substitution upon alkyl halides, or the water from an alkyloxonium cation, primary carbocations are too unstable for SN1 substitution to occur.