A wireless power transfer system (100, 200) comprising an impedance matching circuit, the impedance matching circuit including a variable capacitance device comprising: a first capacitor comprising a first-capacitor first terminal and a first-capacitor second terminal, the first-capacitor first terminal electrically connected to a ground; a first switch comprising a first-switch first terminal, a first-switch second terminal, and a first-switch control terminal, the first-switch first terminal electrically connected to the first-capacitor first terminal, and the first-switch second terminal electrically connected to the first-capacitor second terminal; a second capacitor comprising a second-capacitor first terminal and a second-capacitor second terminal, the second-capacitor first terminal electrically connected to the ground; a second switch comprising a second-switch first terminal, a second-switch second terminal, and a second-switch a control terminal, the second-switch first terminal electrically connected to the second-capacitor first terminal, and the second-switch second terminal electrically connected to the second-capacitor second terminal; and control circuitry (306, 606, 706, 806) coupled with the first-switch control terminal and with the second-switch control terminal, characterised in that the control circuitry is configured to adjust respective capacitances of the first and second capacitors by: in response to detecting a zero voltage condition across the first switch, causing a first control signal to be applied to the first-switch control terminal for a duration of time, wherein the first control signal causes the first switch to close; and in response to detecting a zero voltage condition across the second switch, causing a second control signal to be applied to the second-switch control terminal for the duration of time, wherein the second control signal causes the second switch to close. a first capacitor comprising a first-capacitor first terminal and a first-capacitor second terminal, the first-capacitor first terminal electrically connected to a ground; a first switch comprising a first-switch first terminal, a first-switch second terminal, and a first-switch control terminal, the first-switch first terminal electrically connected to the first-capacitor first terminal, and the first-switch second terminal electrically connected to the first-capacitor second terminal; a second capacitor comprising a second-capacitor first terminal and a second-capacitor second terminal, the second-capacitor first terminal electrically connected to the ground; a second switch comprising a second-switch first terminal, a second-switch second terminal, and a second-switch a control terminal, the second-switch first terminal electrically connected to the second-capacitor first terminal, and the second-switch second terminal electrically connected to the second-capacitor second terminal; and control circuitry (306, 606, 706, 806) coupled with the first-switch control terminal and with the second-switch control terminal, characterised in that the control circuitry is configured to adjust respective capacitances of the first and second capacitors by: in response to detecting a zero voltage condition across the first switch, causing a first control signal to be applied to the first-switch control terminal for a duration of time, wherein the first control signal causes the first switch to close; and in response to detecting a zero voltage condition across the second switch, causing a second control signal to be applied to the second-switch control terminal for the duration of time, wherein the second control signal causes the second switch to close. in response to detecting a zero voltage condition across the first switch, causing a first control signal to be applied to the first-switch control terminal for a duration of time, wherein the first control signal causes the first switch to close; and in response to detecting a zero voltage condition across the second switch, causing a second control signal to be applied to the second-switch control terminal for the duration of time, wherein the second control signal causes the second switch to close.