We numerically investigate the pre-injection mechanism by which pickup ions approaching quasi-perpendicular shocks may be accelerated above the threshold velocity required to begin diffusive shock acceleration. We demonstrate that three distinct mechanisms contribute to the acceleration and that they operate on disjoint subsets of the pickup ion population. We quantitatively compare the efficiency of each mechanism as a function of shock Mach number, pickup ion mass and shock ramp width and show that shock surfing predominates below a critical ramp width. ¿ 2001 American Geophysical Union