Handsheets of post-treated chemithermomechanical pulps (CTMP) and a sample of Whatman paper no. 1 were analyzed using an ESCA spectrometer. All of these samples revealed the presence of the C1, C2, and C3 components in the carbon (1s) peak and the O2 component in the oxygen (1s) peak; the O1 peak component which was present in the other samples was missing from the Whatman paper. Pulp sulfonation produced a reduction in C1 representing the group of carbon not bonded to oxygen, and an increase in C2 with C3 remaining relatively constant. These changes imply reductions in lignin and extractives and a corresponding increase in cellulose content on the fiber surface. Extrapolation of the C1, C2, C1/C2, and O/C curves of CTMP to zero percent sulfonation yielded the same values as those obtained for pure TMP. ESCA analysis allows us to monitor the increase of sulfur content as a function of sulfonation in accordance with results obtained from bulk pulp sulfonate content titration.