A novel ruthenium catalyst, notably a ruthenium-titania catalyst, is disclosed to which is added sufficient rhenium to obtain, at corresponding process conditions, improved activity maintenance in the production of hydrocarbons via carbon monoxide-hydrogen synthesis reactions vis-a-vis a catalyst composition otherwise similar except that it does not contain rhenium. High quality middle distillate fuels, characterized generally as admixtures of linear paraffins and olefins, are formed in a process wherein a feed mixture of carbon monoxide and hydrogen are contacted at reaction conditions over such catalyst.