The authors report results of tests designed to determine whether the plastic deformation of paper is mainly due to an interfibre or intrafibre phenomenon. Their hypothesis is that intrafibre structural deformation is responsible. The thermodynamic behavior of 100 percent rag paper under deformation in the cross-direction was characterized. An initial cooling in accordance with Kelvin's thermoelastic effect is followed by heat generation, and the plastic deformation which occurs is mechanically and thermodynamically irreversible. This can be considered a true plastic response. A previously deformed specimen exhibits the initial cooling over a longer interval. Similar behavior was observed with other types of paper, but other planar materials showed different thermodynamic behavior. Scanning electron microscope examination of paper fibers under strains averaging 3.2 percent elongation showed cracking related to local plastic extensions of the cell wall. Thus, both the thermodynamic and direct SEM analysis indicate that the plastic regime of paper involves a true plastic deformation dominated by intrafibre structural changes.