Size-segregation and stratification (layering of ore into zones of different hydraulic characteristics which results from commercial stacking practices), are commonly believed to affect heap leaching performance. However, little systematically collected data has to date been published on the subject. This first part of the study reports on the effects of deliberate segregation observed under laboratory conditions. An ore sample was segregated on an artificial slope and the geophysical and hydrodynamic properties were determined of the resulting Upper, Middle and Bottom fractions of the segregated ore bed. The same was done for a stack composed of these three fractions layered on top of one another. It was found that the three-layered stack exhibited less favourable bulk density and hydrodynamic conductivity than un-segregated ore (i.e. ore which had not been deliberately segregated). It is therefore suggested that when those parameters are determined for the purpose of heap design, it would be conservative to conduct the tests on a stack of segregated layers of the ore, instead of the un-segregated ore. However no statistically significant difference could be observed in column leaching performance (770 mm high, 153 mm diameter), either between the individual segregation fractions, or between the un-segregated column of ore and the column composed of Bottom, Middle and Upper layers. It therefore follows that segregation cannot account for the differences reported between laboratory column tests and commercial heap leaching results. Furthermore, laboratory column leaching tests cannot emulate commercial scale leaching performance more realistically by merely segregating the ore samples prior to column leaching.