The detection properties of alkanes by proton-transfer-reaction plus switchable reagent ion mass spectrometry (PTR + SRI-MS) were investigated. Alkanes (RH) were commonly detected as R+ by PTR + SRI-MS using NO+ as the reagent ion, and detection sensitivities were comparable to those of aromatics observed by H3O+ ionization. It can be concluded that sensitive online measurement of multiple alkanes is possible by PTR + SRI-MS using NO as the reagent ion.