An apparatus for controlling a radio bearer reconfiguration in a cellular communication system, such as a UMTS communication system, comprises a reconfiguration instigator (301) which initiates a radio bearer reconfiguration for a radio bearer supporting a communication between a base station (105) and a user equipment (107). Rather than using a static worst case estimate, a reconfiguration switch time processor (309) determines a reconfiguration switch time in response to a dynamic parameter of the radio bearer reconfiguration process. In particular, the radio parameter may be a size of a radio bearer configuration message transmitted to the user equipment (107). The reconfiguration switch time is transmitted to the base station by a base station communication processor (311) and to the user equipment (107) by a UE communication processor (313). The base station (105) and the user equipment (107) reconfigure their circuitry to apply the reconfigured parameters from the reconfiguration switch time.