  We follow the diffusive motion of colloidal particles of diameter $d$ in soap
films of varying thickness $h$ with fluorescence microscopy. Diffusion
constants are obtained both from one- and two-particle microrheological
measurements of particle motion in these films. These diffusion constants are
related to the surface viscosity of the interfaces comprising the soap films,
by means of the Trapeznikov approximation [A. A. Trapeznikov, \emph{PICSA}
(1957)] and Saffman's equation for diffusion in a 2D fluid. Unphysical values
of the surface viscosity are found for thick soap films ($h/d > 7$), indicating
a transition from 2D to 3D behavior.
