Collisional transport of particles and energy across magnetic field lines is an active area of plasma theory and experiments, with fundamental questions as to the binary vs collective and short-range vs long-range nature of interparticle collisions. Classical Boltzmann theory [1] describes ion transport in terms of local collisions, with impact parameters less than the cyclotron radius, i.e., r& r c; but it has long been recognized that electron particle and heat transport may involve long-range collisions, with rce ,r& l D [2,3]. In Q machines, experiments with optical tagging of ions [4] have demonstrated classical collisional diffusion [5,6] and shear-induced convection [7]. In fusion plasmas, tritium diffusion and convection coefficients have been obtained from neutron emission, but direct comparison to theory is still ambiguous [8]. Measurements of bulk transport in pure electron plasma have shown enhanced viscosity thought to be due to longrange interactions [9]; and the present experiments are an attempt to quantify these interactions. Here, we measure cross-field test-particle transport in an unneutralized ion plasma, where the cyclotron radius is smaller than the Debye length, i.e., rc ,l D . The quiescent, near-thermal-equilibrium ion column is contained steady state in a Penning-Malmberg trap [10], with no net transport or particle loss. Ions are tagged by spin orientation, and the slow cross-field diffusion of these test particles is accurately measured using laser-induced fluorescence (LIF). This test particle diffusion has been measured over a wide range of density, temperature, and magnetic field, and is generally about ten times faster than predicted by the classical theory of velocity-scattering collisions. Measurements of velocity-space isotropization on the same ion plasmas agree closely with classical theory [11,12], indicating there is no uncertainty in this collision rate. The enhanced test particle diffusion scales essentially as B 22 , and is probably due to long-range “ E 3 B drift” collisions with rc ,r, l D [2,3]. These weak, long-range binary interactions between ions are ultimately limited by velocity-scattering collisions and by shear in the E 3 B rotation of the column.