Gasification technology is an attractive alternative for the thermal treatment of solid wastes, producing a high energy value hydrogen rich syngas. The presence of tar in the produced gas diminishes its quality and potential use in further processes; for this reason the reduction of tar in waste gasification is a major challenge. In this work the pyrolysis/gasification of refuse derived fuel (RDF) from municipal solid wastes, was investigated using a two-stage reaction system with Ni/SiO2 catalysts prepared by a sol–gel method varying the citric acid concentration (CA). The fresh and reacted catalysts were characterised for surface area and pore size distribution, temperature programmed oxidation, and high resolution scanning electron microscopy. The effect of the nickel to citric acid ratio (Ni:CA) was evaluated in terms of the characteristics and performance of the Ni/SiO2 catalysts. The results showed that the prepared Ni/SiO2 catalysts exhibited a relatively high surface area and an increase in pore size distribution as the Ni:CA ratio was increased. The efficiency of the prepared catalysts on tar reduction and hydrogen production was examined during the pyrolysis/gasification of RDF; the results were compared with a blank experiment using a bed of sand. The tar fraction was quantified using gas chromatography/mass spectrometry. A low tar concentration of ~0.2 mgtar/gRDF was attained using the catalysts with Ni:CA ratios of 1:1 and 1:3; additionally a high hydrogen concentration (58 vol%), and low CH4 (2.2 vol%) and C2–C4 concentrations (0.8 vol%), were attained using the catalyst with a Ni:CA ratio of 1:3. A higher tar concentration of ~1.7 mgtar/gRDF was attained using the bed of sand, while the hydrogen production was remarkably decreased. The major tar compounds identified in the tar samples using the Ni/SiO2 catalysts were phenol, cresols, naphthalene, fluorene, and phenanthrene.