A method and unit for filling a disposable electronic cigarette cartridge with a liquid substance; a conveyor feeds the disposable cartridge along a filling path; a coupling device fits the disposable cartridge with a metering chamber, which is placed on top of the disposable cartridge, is of sufficient volume to contain a full measure of the liquid substance, and has a bottom outlet conduit terminating inside the disposable cartridge; a feed device feeds the liquid substance into the metering chamber so that the liquid substance falls by gravity from the metering chamber, along the outlet conduit, into the disposable cartridge; and a removing device removes the metering chamber from the disposable cartridge once all the liquid substance has fallen by gravity from the metering chamber, along the outlet conduit, into the disposable cartridge.