The hydroconversion of tars in the presence of in situ synthesized iron-containing catalysts is studied. Water-soluble iron compounds (FeSO4, Fe(NO3)(3), Fe(COOCH3)(2)) are used as precursors which are introduced into the processed feedstock in the composition of the aqueous phase of an inverted emulsion. The tests are conducted on a flow hydroconversion unit with a vertical hollow reactor. It is found that the catalytic activity of the Fe-containing precursors which, under hydroconversion conditions, form Fe1-xS nanosized particles, in particular, from Fe(COOCH3)(2), is close to the catalytic activity of previously studied nanodispersed MoS2. A disadvantage of Fe-containing catalysts in situ synthesized from water-soluble iron compounds (FeSO4, Fe(NO3)(3), Fe(COOCH3)(2)) is a relatively high yield of condensation products (coke) under hydroconversion conditions.