A method of using a mobile device to authenticate a print medium before completing printing onto the print medium, the mobile device including processing means, a printhead and a sensor, the print medium comprising a substrate, the method comprising the steps of: using the sensor to sense coded data provided on a surface of the substrate; using the processing means to interpret the coded data to authenticate the print medium; and in the event the authentication step is successful, using the printhead to print onto the print medium.