An aerosol-generating system comprising: a liquid storage portion for holding a liquid aerosol-forming substrate; an atomiser in fluid connection with the liquid storage portion; a conductivity sensor (100, 200, 300, 400, 500) arranged to sense the electrical conductivity of liquid aerosol-forming substrate from the liquid storage portion, the conductivity sensor (100, 200, 300, 400, 500) comprising at least two electrodes (104, 106, 204, 206, 304, 306, 404, 406, 504, 506); a power supply; and control electronics (111, 112, 211, 212, 310, 410, 510) configured to: control a supply of power from the power supply to the atomiser for atomising liquid aerosol-forming substrate from the liquid storage portion; control a supply of power from the power supply to the electrodes (104, 106, 204, 206, 304, 306, 404, 406, 504, 506) of the conductivity sensor (100, 200, 300, 400, 500), the supply of power being provided to the conductivity sensor (100, 200, 300, 400, 500) as an alternating voltage; receive one or more measurements indicative of the conductivity of the liquid aerosol-forming substrate from the conductivity sensor (100, 200, 300, 400, 500); and determine the nicotine concentration of the liquid aerosol-forming substrate based on one or more of the measurements from the conductivity sensor (100, 200, 300, 400, 500). a liquid storage portion for holding a liquid aerosol-forming substrate; an atomiser in fluid connection with the liquid storage portion; a conductivity sensor (100, 200, 300, 400, 500) arranged to sense the electrical conductivity of liquid aerosol-forming substrate from the liquid storage portion, the conductivity sensor (100, 200, 300, 400, 500) comprising at least two electrodes (104, 106, 204, 206, 304, 306, 404, 406, 504, 506); a power supply; and control electronics (111, 112, 211, 212, 310, 410, 510) configured to: control a supply of power from the power supply to the atomiser for atomising liquid aerosol-forming substrate from the liquid storage portion; control a supply of power from the power supply to the electrodes (104, 106, 204, 206, 304, 306, 404, 406, 504, 506) of the conductivity sensor (100, 200, 300, 400, 500), the supply of power being provided to the conductivity sensor (100, 200, 300, 400, 500) as an alternating voltage; receive one or more measurements indicative of the conductivity of the liquid aerosol-forming substrate from the conductivity sensor (100, 200, 300, 400, 500); and determine the nicotine concentration of the liquid aerosol-forming substrate based on one or more of the measurements from the conductivity sensor (100, 200, 300, 400, 500). control a supply of power from the power supply to the atomiser for atomising liquid aerosol-forming substrate from the liquid storage portion; control a supply of power from the power supply to the electrodes (104, 106, 204, 206, 304, 306, 404, 406, 504, 506) of the conductivity sensor (100, 200, 300, 400, 500), the supply of power being provided to the conductivity sensor (100, 200, 300, 400, 500) as an alternating voltage; receive one or more measurements indicative of the conductivity of the liquid aerosol-forming substrate from the conductivity sensor (100, 200, 300, 400, 500); and determine the nicotine concentration of the liquid aerosol-forming substrate based on one or more of the measurements from the conductivity sensor (100, 200, 300, 400, 500).