|Planet of Origin:||Venus|
|First Appearance:||The Sulphur Soldiers|
The Second Inspector first encountered the sulphur soldiers in the eponymous serial "The Sulphur Soldiers", set in the 22nd century when Humans tried to conquer Venus. The Inspector, Aiden and Regina land in the middle of a war between the Earth expeditionary forces and the planet's current rulers. Eventually, the humans win thanks to the Inspector's strategy, and the soldiers must evacuate in search of a new planet. They returned in "The Spores of Doom", in which they launch an attack on Earth from Io, the sulfur-rich moon of Jupiter, but they fail once more against the Second Inspector.They are featured briefly in "Circus of Creatures" along with other recurring enemies (their only appearance during the Third Inspector's run).
After their repeated defeats, the Sulphur Soldiers decided against further conflict with the humans and instead established their Sulphur Empire in another arm of the Milky Way galaxy. By the 25th century, their empire comprised numerous planets and was quickly expanding, threatening the development of the Terran Federation. In the serial "Ziggurats of Venus", the Infinity Knights, worried about their increasing influence, send the Fourth Inspector to Venus's past—to a time before the Soldiers became so belligerent—in order to learn their possible weaknesses. The Inspector discovers a pious, pacifist species who are unaware of intelligent life elsewhere in the universe. The Inspector's presence unfortunately attracts the attention of the fiendish Nergal, which sets off a civil war among the Venusians. The resulting the destruction of their temples and development of advanced weaponry ultimately sets them on the path to becoming the familiar warlike species.
In the serial "The Theft of Space", they learn of the Infinity Knights' attempt to stop them and organise a complicated plot to pillage Kayaclasch in retaliation. After the Fourth Inspector uncovers their scheme, Reena implores her father, King Sonacry, for military assistance, and with the aid of his Barbartronian honour guard, the Infinity Knights are able to repulse the Sulphur Soldiers' invasion force.
Their most recent appearance was in the Eleventh Inspector episode "The Great Game", in which a lone Sulphur Soldier wreaks havoc on a Samarkand-bound train on the Trans-Caspian railway in 1888 during the clash between the British and Russian Empires in Central Asia.
Their reptilian nature was not revealed before "Ziggurats of Venus ", in which they first appear without armour. In the majority of stories, they act as soldiers and always wear their special interplanetary combat gear with its characteristic sulphur-dioxide–breathing apparatus, from which they get their name.
When entering their "battle frenzy", their eyes and/or entire bodies would sometimes start to glow, accompanied by, naturally, an increased smell of sulphur.