Polarized infrared absorption spectra of thin, oriented single-crystal slabs of pectolite and serandite were recorded between 4000 and 350 cm (super -1) at 298 and 83 K. The spectra of both minerals show a broad absorption region parallel to the silicate chains (b direction) that is centered around 1000 cm (super -1) , which is interrupted by a transmission window, and which is superimposed by sharp silicate, lattice, and overtone modes. This band is assigned to the OH stretching mode consistent with the alignment of the O-H . . . O hydrogen bond parallel to b and the short O . . . O distance of 2.45-2.48 Aa that was found in previous X-ray structure refinements. At 1396 cm (super -1) (pectolite) and 1386 cm (super -1) (serandite) an OH bending mode is observed in the IR spectra parallel to c. At low temperatures, this mode shifts up to higher frequencies (1403 cm (super -1) at 83 K in pectolite), whereas the down-shift of the OH stretching mode cannot be observed due to the extremely broad band width. The slightly higher energy of the bending mode in pectolite indicates a slightly stronger hydrogen bond with respect to serandite. However, the bond length in serandite is slightly shorter than that in pectolite. An asymmetric O-H . . . O bond is confirmed in pectolite and serandite through comparison with different materials with similar, very strong hydrogen bonds and low-energy OH stretching modes.