In froth flotation, particles that are recovered to the concentrate comprise those attached to bubbles and unattached particles, which include entrained material and particles that have become detached from bubbles in the froth phase, but that are carried into the concentrate. The grade of the particles remaining attached to bubbles at the froth surface is of interest as it offers information on the selective nature of the froth phase and has been shown to indicate the maximum mineral grade that can be obtained for a given feed. In this paper, the grade of particles attached to bubbles at the froth surface is shown as a function of cell feed grade down a bank of cells, and is linked to variation in mineral liberation and flotation kinetics. The grade of attached particles at the froth surface is measured by touch sampling; touching an individual bubble with a microscope slide and collecting the solids. Data from two surveys carried out over the first four cells of the rougher bank at the same copper concentrator have been analysed. It has been shown previously that the surface grade measured in this way does not vary with operating conditions such as air rate, and here it is shown that when plotted as a function of cell feed grade, the attached particle grade exhibits a clear trend that can be modelled empirically by the equation x = 1 - exp (-aG(vf)(b)), where x is the inferred liberation, taken as the attached particle grade at a given feed grade as a fraction of the maximum grade obtainable for the feed to the bank, and G(vf) is the cell feed grade. The shape of the curve, determined by the fitted parameters a and b, is dependent on the mineralogy and liberation characteristics of the feed. This relationship suggests that the variation in flotation kinetics for particles of different liberation in a flotation feed can be linked to attached particle grade. (C) 2015 Elsevier Ltd. All rights reserved.