The Boom Clay in northern Belgium has been studied intensively over recent decades as a potential host rock in the context of disposal of radioactive waste. One of the parameters of interest is the cation exchange capacity (CEC) as it is related to the sorption potential of radionuclides to the clay host rock. In the past, the CEC was determined using various methods on a limited number of samples, leading to significant variations. To constrain the CEC of the Boom Clay better, a sample set covering the entire stratigraphy was measured using the quick copper(II) triethylenetetramine method. Part of the sample set was also measured using the cobalt(III) hexamine method, as a quality control for the results of the former method. In addition, the exchangeable cation population of the Boom Clay was quantified systematically for the first time and these results were compared to the in situ pore-water chemistry, indicating a strong coupling between the pore-water composition and the exchangeable sites of clay minerals.