The continuous-flow syntheses of 1-chlorobutane and 1-bromobutane were achieved in the gas phase starting from 1-butanol, at 130–170 °C and atmospheric pressure. An aqueous mixture of 1-butanol and commercial hydrochloric or hydrobromic acids (37 and 48%, respectively) was fed into a plug-flow catalytic reactor loaded with zinc chloride (ZnCl₂: 5 and 15 wt%) or a phosphonium salt (n-Bu₄P⁺Br⁻: 5 and 15 wt%), both supported on silica gel. Overall yields in butyl halides ranged from 87 to 30%, with weight hourly space velocity (WHSV) up to 0.53 h⁻¹. The onium salt allowed a high reaction selectivity, since no transposition products were observed. In the presence of a molten phase composed of the onium salt, a S_N2 nucleophilic substitution takes place. The reaction mechanism is discussed.