1842
In April, 1842, the Twelfth Regiment having been augmented to an establishment of one lieut.-colonel, twelve captains, fourteen lieutenants, ten ensigns, six staff officers, sixty-seven serjeants, twenty-five drummers, and twelve hundred rank and file, was ordered to be separated into two battalions; the six service companies abroad being termed the First battalion, and the depôt, augmented to six companies, being styled the Reserve battalion.
1843
The depôt was consequently removed from Sunderland to Weedon in May, 1842, and receiving 255 volunteers from other corps, was there organised for foreign service. The reserve battalion embarked from Portsmouth in the "Java" transport for the Mauritius in November, 1842, under the command of Major Sir Robert Douglas, Bart., but was disembarked at the Cape of Good Hope, and remained there until May, 1843, when it proceeded to its original destination.
1847
On the 2nd of November, 1847, Her Majesty's Troopship "Resistance" arrived with the first battalion of the Fifth Fusiliers for the purpose of relieving the first battalion of the Twelfth Regiment, which embarked from the Mauritius on the 16th of December, under the command of Lieutenant-Colonel Patton.
1848
The first battalion arrived at Spithead on the 1st of March, 1848, and disembarked on the 3rd of March at Portsmouth, where it was joined by the depôt company from the Isle of Wight. The reserve battalion, after being completed by the transfer of effective men from the first battalion, continued at the Mauritius.