The potential effect of near-field alkaline perturbation in clayrocks on Cs retention was explored. Batch experiments were conducted to measure Cs sorption on Callovian-Oxfordian rock samples in alkaline perturbed conditions. Experimental results evidenced a marked effect of alkaline perturbation on the Cs retention properties of Callovian-Oxfordian rock samples. Using a modelling approach, this effect could be attributed to the competition of K+, originating from the alkaline solution, for Cs sorption sites on illite surfaces. Experimental results also showed that Cs sorption tends to increase with time. Additional experiments on a mineralogical assemblage representative of a Callovian-Oxfordian rock/concrete interface after long term alkaline perturbation showed that this mineralogical assemblage has a similar efficiency for Cs uptake as the intact clay rock, showing that long term alkaline perturbation of clayrocks and resulting mineralogical changes do not affect the barrier performance considerably with regard to Cs retention.