An image forming apparatus comprising: image forming means configured to form toner images on a series of recording media based on image data; fixing means configured to thermally fix the toner images onto the recording media; and stacking means configured to receive the series of recording media from the fixing means and to stack the received recording media one upon another; characterised in that the image forming apparatus further comprises: control means operable, before a further recording medium of the series is stacked by the stacking means on top of one or more preceding recording media such that a bottom surface of said further recording medium comes into contact with a top surface of an uppermost one of the preceding stacked recording media of the series, to calculate, based on the image data, a first measure of an amount of toner in the toner image formed on said top surface of said uppermost one of the preceding stacked recording media and a second measure of an amount of toner in the toner image formed on said bottom surface of said further recording medium and to employ the first and second measures to control a timing at which said further recording medium is stacked on top of said stacked recording medium by said stacking means. image forming means configured to form toner images on a series of recording media based on image data; fixing means configured to thermally fix the toner images onto the recording media; and stacking means configured to receive the series of recording media from the fixing means and to stack the received recording media one upon another; characterised in that the image forming apparatus further comprises: control means operable, before a further recording medium of the series is stacked by the stacking means on top of one or more preceding recording media such that a bottom surface of said further recording medium comes into contact with a top surface of an uppermost one of the preceding stacked recording media of the series, to calculate, based on the image data, a first measure of an amount of toner in the toner image formed on said top surface of said uppermost one of the preceding stacked recording media and a second measure of an amount of toner in the toner image formed on said bottom surface of said further recording medium and to employ the first and second measures to control a timing at which said further recording medium is stacked on top of said stacked recording medium by said stacking means. control means operable, before a further recording medium of the series is stacked by the stacking means on top of one or more preceding recording media such that a bottom surface of said further recording medium comes into contact with a top surface of an uppermost one of the preceding stacked recording media of the series, to calculate, based on the image data, a first measure of an amount of toner in the toner image formed on said top surface of said uppermost one of the preceding stacked recording media and a second measure of an amount of toner in the toner image formed on said bottom surface of said further recording medium and to employ the first and second measures to control a timing at which said further recording medium is stacked on top of said stacked recording medium by said stacking means.