Two series of iron-pentasil molecular sieve catalysts were synthesized. One series consisted of HZSM- 5 type zeolites with different silicon/aluminium ratios impregnated with ferric nitrate, and the other included ferrisilicates with different silicon/iron ratios. The catalytic tests on syngas conversion were effected at 0.7-2.1 MPa at between 240-300°C in a flow system with a fixed bed reactor. The characterization of the solids shows that in the impregnated series, the iron was transformed into oiron and carbide phases by means of treatment with hydrogen, carbon monoxide and by the effect of the syngas under the reaction conditions. On the other hand, in the ferrisilicate series, the iron retains the ionic form [iron (III) -iron (II) 1, regardless of the treatment, thereby indicating its incorporation into the structure of the solid. The catalytic behaviour of both series also differs. The solids produced by impregnation were seen to produce mostly hydrocarbons in the petrol range with the expected selectivity properties, whereas the ferrisilicates were selective towards the formation of i-C,.