Visible craze initiation stress has been measured for a wide range of linear and branched monodisperse polystyrenes (PS) soaked in diethylene glycol. Results show that, for a given time under stress, craze initiation in linear PS is disentanglement-dominated below a critical molar mass and chain scission-dominated above it. Branched monodisperse PS behaves similarly, with the relevant molar mass in this case being the span molar mass. Disentanglement craze initiation stress is found to vary linearly with log molar mass and log time. These observations can be explained in terms of Eyring-type stress acceleration of the process of chain retraction, required to achieve the entanglement loss necessary for creation of craze fibril surfaces. A single effective activation volume of 1.8 nm(3) accounts for the dependence of crazing stress on molar mass, time, and temperature under uniaxial tensile stress, both as observed in the present data and in a previous study of rate/temperature dependence.