-  The impact of particle size on colloid transport was explored in two saturated, naturally discrete fractured chalk cores, with equivalent hydraulic apertures of 183 and 380 μm. Tracer experiments were carried out using negatively charged fluorescent latex microspheres (FluoSpheres®: 0.02, 0.1, 0.2, and 1.0 μm diameter) as well as Li+ and Br− as the soluble tracers. In both fractures, FluoSpheres exhibited earlier arrival times than the solutes and a complete lack of tails in their breakthrough curves, proving that their transport is advection-dominant. In all experiments the 0.2-μm FluoSpheres were recovered to a much greater extent than the 0.02-μm FluoSpheres and to a slightly greater extent than the 1.0-μm FluoSpheres. Similarly, the highest maximum C/C0 values were found for 0.2 μm, then for 1.0 μm, while the maximum C/C0 values for the 0.02-μm colloids were significantly lower. The insignificant contribution of settling relative to Brownian motion (diffusion) as an efficient deposition mechanism was demonstrated for all sizes of FluoSpheres in both fractures.