The liquid carrier in a Fischer-Tropsch synthesis slurry reactor system is periodically or continually separated and subjected to cracking and isomerization in the presence of suitable catalysts. The treated carrier is returned to the reactor system and the accumulation of high viscosity paraffin in the reactor slurry is minimized. Suitable catalysts include a mixture of cracking and isomerization catalysts. Zeolite Beta is the novel constituent of the catalyst system.