We experimentally assess the validity of the symmetric decomposition of Mueller matrices [R. Ossikovski, J. Opt. Soc. Am A 26, 1109-1118 (2009)] into a sequence of five factors consisting of a diagonal depolarizer between two retarder and diattenuator pairs. The raw data were Mueller images of combinations of polarization components which were individually measured and then assembled in different combinations. The possibility to recover all the elements is discussed, including the experimentally relevant cases of "degenerate" depolarizers, with two equal eigenvalues, which were not explicitly considered in the general theory.