A method is provided of shutting down an operating three-phase slurry bubble column reactor (10) having downwardly directed gas distribution nozzles (30) submerged in a slurry body (19) of solid particulate material suspended in a suspension liquid contained inside a reactor vessel (12), with the gas distribution nozzles (30) being in flow communication with a gas feed line (26) through which gas is being fed to the gas distribution nozzles (30) by means of which the gas is injected downwardly into the slurry body (19). The method includes abruptly stopping flow of gas from the gas feed line (26) to the gas distribution nozzles (30) to trap gas in the gas distribution nozzles (30) thereby to inhibit slurry ingress upwardly into the gas distribution nozzles (30).