An inductive power transfer system (1) for coupling a power source to a load across an air gap (11) is disclosed. The system (1) comprises a primary unit (3) associated with a host platform and a secondary unit (5) arranged to receive power transmitted inductively from the primary unit (3). The primary unit (3) includes a phase detection circuit (21) configured to detect phase changes in a signal in the primary unit (3) indicative of changes in an operating condition within the secondary unit (5), and a drive circuit (17). The drive circuit (17) is configured to adjust the power level transmitted to the secondary unit (5) depending on the detected phase.