Synthesis of a series of Sm2Sn2O7 pyrochlore catalysts by a sol-gel-like method has been developed. The preparation from the oxides gives stoichiometric pyrochlore, unlike the result obtained when starting from chlorinated salts (SmCl3 or SnCl2), which form tin-deficient pyrochlores. A spectroscopic study of the behaviour of the starting oxides or chlorides in propionic acid, to generate a series of precursors, has been carried out, particular attention being focused on the precursors generated by SnCl2 in propionic acid using Sn-119 NMR and FTIR spectroscopy. The formation of SnCl2(C2H5COO)(2) as the main precursor is responsible for the tin deficiency observed in the pyrochlore structure. The catalytic performance in oxidative coupling of methane is directly correlated with the tin deficiency in the bulk of the pyrochlore. The work highlights the importance of controlling precisely the synthesis of the catalysts.