A method is presented of allocating communication time slots contained in repeating frames for communication between an inductive wireless power transmitter and at least two inductive wireless power receivers, wherein the power transmitter and the power receivers are arranged to communicate by means of modulation and demodulation of an inductive power signal. The transmitter sends synchronization messages marking the start of the communication time slots and the frames, and messages indicating if a time slot is unallocated. A receiver may send, during an unallocated time slot, send a message to the transmitter requesting allocation of the unallocated communication time slot. The transmitter subsequently sends messages to indicate if the communication was successfully received, and if the allocation request is granted.