The aim of this work is to determine the apparent rate constants for the n-C-25 thermal cracking and, then, to study pressure effect on these rate constants so as to evaluate n-alkane stability in high-pressure/high-temperature deep pro spe cts. Pyrolyses were carried out in anhydrous do sed system (gold vessels) during times ranging from 1 to 360 h under isothermal conditions (325-425 degrees C) and various pressures (120, 400, and 800 bar) in the range of those occurring in petroleum systems. Results show that degradation of n-C-25 follows a first-order kinetics under isothermal conditions at 120 bar. The main chemical class produced along increasing severity is the saturated hydrocarbons ranging from C-10 to C-22 (80 wt %). The gas contribution is very low (<2 wt %) up to 85 wt % conversion. On the basis of the linearity of the Arrhenius plot, accurate kinetic parameters were calculated (E = 68.2 kcal/mol and A = 6.1 x 10(17) s(-1)). An extrapolation of these kinetic parameters to geological conditions shows that n-C-25 begins to be measurably degraded above 180 degrees C for residence times higher than 10 million years. This means that a reservoired oil enriched in n-alkanes will be stable in very high temperature conditions. When this oil begins to degrade, condensate is mainly produced and will be measurably degraded only above 200 degrees C during similar residence times. Pressure effect increases the thermal degradation rate of n-C-25 between 120 and 400 bar then decreases it between 400 and 800 bar down to its value at 120 bar. These results are in a good agreement with those already published (Fabuss, B. M.; et al. In Advances in Petroleum Chemistry and Refining; Me Ketta, J., Jr., Ed.; Wiley and Sons : New York, 1964; Vol. 5, Chapter 4, pp 156-201) for thermal degradation of n-C-6 and n-C-7 saturated hydrocarbons in the same range of pressure. Nevertheless, the relative increase of the degradation rate for n-C-25 reaching a maximum around 1.7 for 400 bar, i.e., for an average depth of 4000 m and a temperature of 140 degrees C, the pressure effect is not sufficient to allow a measurable degradation in these geological conditions.