Abstract: A capsule holder for retaining a mixing capsule on a rotary mixing device includes a carrier plate 10, an actuator disk 11 rotatable relative to the plate, and two retaining arms 12, 13 for clamping the capsule from opposite sides, the retaining arms having driving pins 22 engaging spiral cam slots 26 of the actuator disk 11. By rotating the actuator disk 11, the retaining arms 12, 13 are moved away from, or toward, each other. The thus constituted gearing is self-locking in one direction of force transmission so that centrifugal forces exerted on the retaining arms 12, 13 during rotation cannot move the arms apart. On the other hand, the retaining arms 12, 13 are opened with little effort by rotating the actuator disk 11.