<p>Understanding the coexistence of oil and water is crucial for improving oil pollution control. This study aimed to study the infiltration relationship between the water and oil phases and soil and to analyze the distribution status of the two substances in soil. To explore the coexistence of water and oil molecules in contaminated soil, this study used diesel as a representative petroleum hydrocarbon and examined the effects of diesel pollutants on the microstructure of clay. Soil samples from the Nanjing region in China and 0# diesel were used as contaminants in artificially prepared diesel-contaminated soils with different levels of oil and water content. The adsorption characteristics of soils for diesel and water, the coexistence state of oil and water within the soil, and the soil microstructure before and after pollution were studied using nuclear magnetic resonance, high-power electron microscopy, and scanning electron microscopy. The findings indicated that the adsorption capacity of soil for oil exceeded that for water. As the oil content increased, the oil molecules gradually displaced the water molecules surrounding the soil particles. Nuclear magnetic resonance tests demonstrated that diesel preferentially occupied larger pores within the clay, in contrast with water. The scanning electron microscopy results revealed that the diesel contamination modified the soil microstructure. Uncontaminated soil primarily displayed a flaky structure; however, as the oil content increased, a noticeable "aggregation" of particles occurred in contaminated soil. Consequently, the soil pores evolved from intergranular to inter-aggregated structures. This study offers a theoretical foundation for analyzing alterations in the external characteristics of soil and provides insights into the remediation of oil-contaminated soil.</p>