A method of operation for an inductive power transfer system comprising a power transmitter arranged to generate a wireless power signal for a power receiver, the inductive power transfer system supporting two-way communication between the power transmitter and the power receiver based on modulation of the power signal, the method comprising: the power receiver initiating a mandatory configuration phase by transmitting a signal strength package to the power transmitter; the power transmitter and the power receiver operating (505, 507) the mandatory configuration phase wherein a first set of power transfer operating parameters are selected for the power transmitter and the power receiver; the power receiver transmitting a request to enter a requested negotiation phase; characterized in that the method further comprises the power transmitter acknowledging the request to enter the requested negotiation phase by transmitting an acknowledgement to the power receiver, the acknowledgement being indicative of an accept or rejection of the request to enter the requested negotiation phase; the power transmitter entering the requested negotiation phase in response to receiving the request to enter the requested negotiation phase; the power receiver entering the requested negotiation phase in response to receiving the acknowledgment from the power transmitter if the acknowledgement is indicative of an accept of the request to enter the requested negotiation phase; the power receiver and power transmitter determining (513, 515) a second set of operating parameters by performing the requested negotiation phase; wherein, when in the negotiation phase (513, 515), the power receiver and power transmitter determine the second set of power transfer operating parameters in a number of negotiation cycles, each negotiation cycle comprising the power receiver transmitting a message specifying at least one of the power transfer operating parameters and the power transmitter responding with a message accepting or rejecting the at least one power transfer operating parameter. the power receiver initiating a mandatory configuration phase by transmitting a signal strength package to the power transmitter; the power transmitter and the power receiver operating (505, 507) the mandatory configuration phase wherein a first set of power transfer operating parameters are selected for the power transmitter and the power receiver; the power receiver transmitting a request to enter a requested negotiation phase; characterized in that the method further comprises the power transmitter acknowledging the request to enter the requested negotiation phase by transmitting an acknowledgement to the power receiver, the acknowledgement being indicative of an accept or rejection of the request to enter the requested negotiation phase; the power transmitter entering the requested negotiation phase in response to receiving the request to enter the requested negotiation phase; the power receiver entering the requested negotiation phase in response to receiving the acknowledgment from the power transmitter if the acknowledgement is indicative of an accept of the request to enter the requested negotiation phase; the power receiver and power transmitter determining (513, 515) a second set of operating parameters by performing the requested negotiation phase; wherein, when in the negotiation phase (513, 515), the power receiver and power transmitter determine the second set of power transfer operating parameters in a number of negotiation cycles, each negotiation cycle comprising the power receiver transmitting a message specifying at least one of the power transfer operating parameters and the power transmitter responding with a message accepting or rejecting the at least one power transfer operating parameter.