The silanol acidity of Waters Resolve C 1 8 , Waters Resolve silica, Waters Symmetry C 1 8 , Waters Symmetry silica, Waters XTerra MS C 1 8 and underivatized XTerra columns has been measured from the retention of LiNO 3 with a methanol/water (60:40) mobile phase buffered to different pH values. The Li + cation is retained by cationic exchange with the background cation of the mobile phase (Na + ) through the ionized silanols. The number of active silanols increases in the order: XTerra MS C 1 8 Symmetry C 1 8 <underivatized XTerra Resolve C 1 8 <Resolve silica~Symmetry silica. XTerra MS C 1 8 does not present any residual silanol acidity up to s s pH 10.0 (pH in 60% methanol) as measured by LiNO 3 . The underivatized XTerra packing and Symmetry C 1 8 present active silanols only at s s pH values higher than 7.0. For the other three columns, two different types of silanols with different acidity ( s s pK a values about 3.5-4.6 and 6.2-6.8, respectively) have been observed. Symmetry C 1 8 shows evidence of the presence of active basic sites that retain NO 3 - by anionic exchange.