The origin of phase contrast in tapping-mode atomic force microscopy has been investigated using two complementary scanning probe microscopy techniques, atomic force microscopy and shear force microscopy, which can be classified as a transverse dynamic force microscopy. The sample chosen for this study was Nafion, and specifically the membrane in different hydration states by virtue of its cation form. Differences in probe-sample adhesion throughout a sample, caused by an inhomogeneous distribution of surface water, were an important phase-contrast mechanism. A new variant in three-dimensional force imaging, phase-volume imaging has been a useful tool in the interpretation of phase contrast. With the use of transverse dynamic force microscopy, approach curves were obtained while the frequency spectrum around resonance was measured. This enabled the damping of the probe oscillation amplitude and the shift in its resonant frequency to be decoupled. Knowing the true oscillation amplitude of the probe, it was also possible to determine quantitatively the elastic and dissipative parts of the probe-sample interaction. Distinct regimes were found at different probe-sample separations.