Experimental equilibrium measurements, its AN value (Gutmann-Beckett method) as well as quantum-chemical calculations all indicate that the Lewis acidity of  is slightly lower than that of  and significantly reduced compared to .  forms a strong Lewis adduct with water, which was shown to be a strong Brønsted acid having an acidity comparable to hydrochloric acid (in acetonitrile).
ɛkspˌɛɹɪmˈɛntəl ˌiːkwɪlˈɪbɹiəm mˈɛʒɚmənts, ɪts æn vˈæljuː ɡˈʌtmənbˈɛkɪt mˈɛθəd æz wˈɛl æz kwˈɔntəmkˈɛmɪkəl kˌælkjʊlˈeɪʃənz ˈɔːl ˈɪndᵻkˌeɪt ðætðə lˈuːɪs æsˈɪdᵻɾi ʌv ɪz slˈaɪtli lˈoʊɚ ðɐn ðæt ʌv ænd sɪɡnˈɪfɪkəntli ɹᵻdˈuːst kəmpˈɛɹd tuː .  fˈɔːɹmz ɐ stɹˈɔŋ lˈuːɪs ɐdˈʌkt wɪð wˈɔːɾɚ, wˌɪtʃ wʌz ʃˈoʊn təbi ɐ stɹˈɔŋ bɹˈɑːnstᵻd ˈæsɪd hˌævɪŋ ɐn æsˈɪdᵻɾi kˈɑːmpɚɹəbəl tə hˌaɪdɹəklˈoːɹɪk ˈæsɪd ɪn ˈæsɪtˌɑːnaɪtɹˌaɪl.