A process for increasing the resistance to water of a chromatographic sorbent layer coated onto a support and comprising a binder of (a) salt(s) of (a) polyacrylic acid(s) and/or polymethacrylic acid(s), comprises treating the coated sorbent layer with an acid and then washing and drying the acid treated layers. The thus-prepared chromatographic materials are not only highly water resistant but also have an advantageously low impurity content.