Deep desulfurization of hydrocarbon feeds containing undesired organosulfur compounds to produce a hydrocarbon product having low levels of sulfur, i.e., 15 ppmw or less of sulfur, is achieved by flashing the feed at a target cut point temperature to obtain two fractions. A low boiling temperature fraction contains refractory, sterically hindered sulfur-containing compounds, which have a boiling point at or above the target cut point temperature. A high boiling temperature fraction, having a boiling point below the target cut point temperature, is substantially free of refractory sulfur-containing compounds. The high boiling temperature fraction is contacted with isomerization catalyst, and the isomerized effluent and the low boiling temperature fraction are combined and contacted with a hydrotreating catalyst in a hydrodesulfurization reaction zone operating under mild conditions to reduce the quantity of organosulfur compounds to an ultra-low level.