The funnel method presented as a technique for investigating dynamic surface properties at the liquid/air interface consists of a hydrophobic funnel which is moved through a liquid interface in order to compress or expand its area isotropically. A counter-body compensates the displaced liquid by moving in the direction opposite to the funnel movement allowing for a constant liquid phase boundary level. Relaxation processes induced by the funnel movement are monitored by means of the surface tension response measured by a Wilhelmy plate right after expansion/compression of the interface. Studies are preformed with the non-ionic surfactant dodecyl dimethyl phosphine oxide. The relaxation curves depend on the direction and amount of the interfacial deformation. The experimental data are interpreted by a diffusion relaxation theory. Differences between experiment and theory are explained by changes in the effective area during compression or expansion.