Objective: The hydrophobicity of tropical peat soil was assessed on different oil palm plantations in Labuan Batu District, North Sumatra, Indonesia. Methodology: The level of hydrophobicity was measured by the Water Drop Penetration Time (WDPT) method and using Fourier transform infrared (FTIR) spectroscopy for analysis of hydrophobic and hydrophilic functional groups in this soil. The critical level of water content that caused hydrophobicity in peat soil was obtained from the exponential function relationship between soil water content and the probability of hydrophobicity. Results: The critical water content determined at which the probability of occurrence of hydrophobicity stands at 60-80%. The mean critical water content of peat soil from the Panai Jaya Plantation (PAJ; 247.06-265.3% w/w) was higher than from the Meranti Paham Plantation (MEP; 151.41-169.39% w/w) indicating PAJ soils will be more susceptible to hydrophobicity than those of MEP. When dryness occurs, the hydrophobic components of peat soil at PAJ cause those soils to become hydrophobic more rapidly than those of MEP. The ratio of hydrophobic/hydrophilic components and water content of peat soil generates an exponential relationship, showing that decreased soil water content increases the ratio of production of hydrophobic/hydrophilic components, thus increasing the hydrophobicity of peat soil. Conclusion: The ratio of hydrophobic/hydrophilic components on hydrophobic peat soil of PAJ ranged 0.457-0.511 (Sapric soil) and 0.494-0.509 (Hemic soil) and hydrophobic peat soil of MEP ranged 0.490-0.508 (Sapric soil) and 0.491-0.505 (Hemic soil). A decrease in the soil water content will result in an increase in this ratio and in the hydrophobicity of peat soil.