The sorption of alkaline earth cations Ca2+, Sr2+, Ba2+, and Ra2+ was investigated on Na-converted biotite, the main sorbing mineral in granitic bedrock at the nuclear waste disposal site in Olkiluoto, Finland. Batch sorption experiments were conducted under CO2-free conditions by varying the concentration of the alkaline earth cation (Ca2+, Sr2+ and Ba2+ 10−8 M–10−2 M; Ra2+ 10−10 M–10−7 M) in 0.01 M, 0.1 M and 1.0 M NaClO4 solutions buffered to pH 8. Additional experiments were conducted on four Olkiluoto bedrock reference groundwater solutions. The retention of alkaline earth cations on biotite decreased upon their increasing concentration in the liquid phase and on increasing NaClO4 solution concentration. The Kd values in NaClO4 solutions and in glacial melt water (ionic strength 0.12... mmol/L) and fresh groundwater (4.3 mmol/L) systematically decreased in the order Ra2+ ≥ Ba2+ > Sr2+ ≥ Ca2+. No apparent sorption occurred except a slight sorption activity for Ba2+ in brackish (91.4 mmol/L) and saline (515 mmol/L) reference waters. The sorption of Ca2+, Sr2+, Ba2+, and Ra2+ was modelled in 0.01 M, 0.1 M, and 1.0 M NaClO4 solutions and in Olkiluoto reference groundwaters using a mechanistic three-site cation exchange model previously developed for Cs+ sorption on biotite. The model gave a generally good fit with experimental results in NaClO4 solutions and in glacial melt water and fresh groundwater reference solutions. The model, however, gave overestimated values because sorption was not detected in the experimental sorption tests for Ca2+ in 1 M NaClO4 solutions and for the other alkaline earth metals in brackish and saline reference waters.