The effect of nitrogen and oxygen plasma surface treatments on the compressive strength of PBO fibers has been studied. To this end, the nucleation and propagation of compression-induced kink bands was carefully monitored by means of in situ bending tests inside a scanning electron microscope. The micromechanisms of deformation were identical irrespective of fiber surface condition (either as-received or modified by plasma) but the critical stress necessary to induce irreversible damage in compression in the nitrogen-plasma treated fibers was 40% higher than in the as-received fibers. This improvement occurred without any reduction in the fiber tensile properties. The source of this behavior is discussed in the light of the morphological and chemical changes induced by the plasma treatments on the fiber surface, as studied by AFM and XPS. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2052-2063, 2012