Abnormal profiles of organic matter maturation in overpressured sequences, which express the retardation of maturation, are a worldwide phenomenon; however, the cause of this retardation has not been explained satisfactorily. Clearly, the different types of pressure in an overpressured regime must be identified in order to determine their role in the retardation of organic matter maturation. Differential pressure is shown to have a marked influence on the maturation of organic matter through its effects on the thermal properties of the source-rock system. Vitrinite reflectance data from several wells in basins offshore NW Europe have been used to calibrate source-rock maturation against differential pressure and "effective temperature " (i. e. observed temperature in an overpressured sequence corrected to the temperature at the same depth in a normally-pressured sequence). High correlation coefficients with vitrinite reflectance have been measured for both relationships. Differential pressure is a determining factor in porosity reduction. Temperature and porosity affect the thermal conductivity of a rock in both overpressured and normally-pressured sequences according to established formulae. Cumulative heat flow, the product of observed temperature and thermal conductivity, is an important value which is affected indirectly by differential pressure. A close linear correlation between cumulative heatflow and vitrinite reflectance is proved. Cumulative heat flow can be related to thermodynamic properties of source-rock systems, and it can be compared to the enthalpy of a system or the internal energy of a reaction. In overpressured sequences, conditions resemble those of constant pressure and constant volume reactions. As a result, cumulative heatflow can be taken as a measure of the final state of a reaction, corresponding in this discussion to the level of vitrinite reflectance. So it can be used as a guide to the state of source-rock maturation in both normally-pressured and overpressured sequences.