Porosity is a fundamental parameter that must be correctly determined in order to relate physical property, hydrologic, and chemical flux studies to natural systems. Traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant. To produce a true porosity distribution such data must be corrected to account for the H2O residing in smectite interlayers which can make up to 25% of the total hydrated mineral mass. Such H2O is easily removed from the mineral by oven drying and/or exposure to low humidities. Standard physical property measurements can be corrected, provided the weight percent smectite in the sediment is known and the interlayer H2O content of the mineral can be estimated. We illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite-rich Barbados abyssal plain and accretionary wedge sediments from Ocean Drilling Program Leg 110, Sites 671 and 672.