  We present a new method of deriving a boundary condition at a thin membrane
for diffusion from experimental data. Based on experimental results obtained
for normal diffusion of ethanol in water, we show that the derived boundary
condition at a membrane contains a term with the Riemann--Liouville fractional
time derivative of the $1/2$ order. Such a form of the boundary condition shows
that a transfer of particles through a thin membrane is a `long memory
process'. Presented method is an example that an important part of mathematical
model of physical process may be derived directly from experimental data.
