The phase shift in amplitude-controlled dynamic atomic force microscopy (AFM) is shown to depend on the cantilever-sample tilt angle. For a silicon sample and tip the phase shift changes by nearly 15º for a change in tilt angle of 15º. This contribution to the phase results from the oscillating tip's motion parallel to the surface, which contributes to the overall energy dissipation. It occurs even when the measurements are carried out in the attractive regime. An off-axis dynamic AFM model incorporating van der Waals attraction and a thin viscous damping layer near the surface successfully describes the observed phase shifts. This effect must be considered to interpret phase images quantitatively. © 2004 American Institute of Physics.