Preferential flow and transport through macropores affects plant water use efficiency and enhances leaching of agrochemicals and the transport of colloids, thereby increasing the risk for contamination of groundwater resources. As part of the Danish Pesticide Leaching Assessment Program this study investigated effects of soil compaction and organic carbon content on macroporosity and preferential flow and transport. More than 150 undisturbed soil cores were extracted from two field sites in Denmark. While both sites exhibited loamy soil textures, one site had significantly higher organic carbon contents. Tritium breakthrough experiments were conducted at constant flow rates and obtained breakthrough curves were analyzed for 5% tracer arrival time and dispersivity. Both soils showed strong correlations between soil compaction, expressed in terms of bulk density, and preferential flow. Interestingly, the relationships between bulk density and tritium transport parameters were markedly different for the two soils although the relationship between bulk density and macroporosity was nearly identical. The difference was likely caused by the higher organic carbon content of one soil leading to a more pronounced pipe-like pore system with well-aligned macropores.