The paper presents both macroscopic and microstructural evidence of the water sensitivity of a low porosity clayey rock from Northern Spain. Particular emphasis is focused on the effects of water sensitivity at micro level. Results obtained from Environmental Scanning Electron Microscopy (ESEM) using the common procedure of sample preparation show that this procedure of sample preparation is not enough to evaluate in a proper way the irreversible changes induced at micro level by hydraulic effects. An alternative procedure is described, which seems to improve the evaluation of rock degradation at micro scale.