In this study the catalytic effects of dolomite and silica on biomass tar decomposition were investigated. The concentration of naphthalene is of particular interest since it is the most difficult compound to decompose when dolomite is used as catalyst. The two catalysts were tested in different combinations to see whether synergetic effects on the cracking of naphthalene could be found. Thermal and catalytic cracking were carried out at 700-900degreesC under ambient pressure in a fixed bed reactor using a tar-rich gas obtained from pyrolysis of different biomass materials. Characterisation of light components of tars using the solid phase adsorption method was also performed. Experimental results indicate that when a pure silica is placed in a layer above the dolomite, considerably less naphthalene and total light tar remains after cracking.