A process for the hydroisomerization of paraffinic hydrocarbons employing a catalyst composed of a noble metal, alumina and chlorine. The catalyst is prepared by treating a composite of noble metal and alumina with an aluminum compound such as an inorganic or organic salt of aluminum, preferably aluminum nitrate, calcining the treated composite and thereafter contacting the treated composite with a conventional chloride activating agent. By treating and calcining the composite with an aluminum salt, the amount of noble metal retained on the catalyst's surface during chloride activation is maximized enabling high conversions of normal paraffins to isoparaffins to be realized.