Extraction of potato cell-wall material with solutions of increasing strength of alkali yielded a xyloglucan-rich fraction which was further purified by anion-exchange chromatography and treatment with α-amylase and endogalactanase. Methylation analysis indicated that the purified xyloglucan contained a high percentage of unsubstituted glucosyl residues compared to, for instance, apple xyloglucan, and equal amounts of Xyl-(1 → 6)-, Gal-(1 → 2)-Xyl-(1 → 6)-, and Ara-(1 → 2)-Xyl-(1 → 6)-sidechains. This xyloglucan was degraded with endoglucanase (endoV), purified from Trichoderma viride. The resulting digest was fractionated by BioGel P-2 chromatography, followed by preparative high-performance anion-exchange chromatography of the pentamer to nonamer fractions. The purified oligosaccharides were characterized by monosaccharide analysis, mass spectrometry, and degradation with an exoglucanase. Degradation of potato xyloglucan by another endoglucanase (endoIV) of Trichoderma viride yielded a different set of products. EndoIV released predominantly oligosaccharides with two unbranched glucosyl residues at the reducing terminus, whereas endoV also released products containing unbranched glucosyl residues on both ends of the molecule. A difference in the mode of action of endoglucanases with xyloglucan-degrading activity is demonstrated.