In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, transition to cooperative deformation of multiple effective network chains via bond torsions. We introduce here the activation number, n, the ratio of the Eyring activation volume, V*, to the network chain volume, V-c, as a measure of the number of network chains, 'activated' at yield for cooperative deformation. We find that n similar to 2-4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks, over a range of network junction functionalities and V*. This first reported analysis of yield at various strain rates, of the octafunctional epoxy, SU-8, is via our compression experiments on fabricated micropillars. Molecular validation of our bulk density based analysis to determine V-c, is achieved by mapping to the eight-chain network model of Arruda and Boyce. We thus identify the activation number as a universal molecular descriptor of yield in densely crosslinked glassy thermosets. (C) 2015 Elsevier Ltd. All rights reserved.