Most bathymetric profiles of oceanic trenches show a well-defined outer rise seaward of the trench. The widths of the observed outer rises are used with an elastic flexure model to determine the effective elastic thickness (EET) of the oceanic lithosphere as a function of the age of the lithosphere being subducted. A model for the EET is proposed in which its upper boundary is taken at a prescribed depth and its lower boundary at a prescribed temperature. It is assumed that the base temperature is the temperature at which elastic stresses are relaxed by dislocation creep on geological time scales. Using a laboratory-derived creep law this relaxation temperature is found to be 700*m*100 ¿C; this result is relatively insensitive to time and stress levels. Using accepted thermal models for the oceanic lithosphere, this relaxation temperature gives values of the EET which are in good agreement with the observations.