Already in 2012, Blom et al. reported (Nature Materials 2012, 11, 882) in semiconducting polymers on a general electron-trap density of approximate to 3 x 10(17) cm(-3), centered at an energy of approximate to 3.6 eV below vacuum. It was suggested that traps have an extrinsic origin, with the water-oxygen complex [2(H2O)-O-2] as a possible candidate, based on its electron affinity. However, further evidence is lacking and the origin of universal electron traps remained elusive. Here, in polymer diodes, the temperature-dependence of reversible electron traps is investigated that develop under bias stress slowly over minutes to a density of 2 x 10(17) cm(-3), centered at an energy of 3.6 eV below vacuum. The trap build-up dynamics follows a 3(rd)-order kinetics, in line with that traps form via an encounter between three diffusing precursor particles. The accordance between universal and slowly evolving traps suggests that general electron traps in semiconducting polymers form via a triple-encounter process between oxygen and water molecules that form the suggested [2(H2O)-O-2] complex as the trap origin.