The Thursday night programme, reuniting '90s bands B*Witched, Liberty X, 5ive, 911, The Honeyz and Atomic Kitten, climbed to 1.12m - a 4.9% share - adding 180,000 on +1.
Not only was The Big Reunion the most-watched multichannel offering of the day, it helped ITV2 finish third in the 9pm slot in front of BBC Two, Channel 4 and Channel 5.
Only BBC One with Silent Witness (5.14m/22.3%) and Martin Clunes: Heavy Horsepower (3.86m/16.7%) on ITV (+1: 286k) could better the ITV2 show, which began with under a million viewers last week.
5's Winter Road Rescue actually tied with The Big Reunion with 1.12m (4.9%), although the latter attracted a higher peak and pipped Road Rescue by a tiny margin of ten thousand viewers.
Meanwhile, How to Build a Bionic Man delivered 1.02m (4.4%) to Channel 4 (+1: 161k), and Dave Gorman's The Genius of Invention could only interest 979k (4.3%) on BBC Two.
Location, Location, Location continued to perform well at 8pm with 2.01m (8.8%) for Channel 4 (+1: 234k), while BBC Two's The Planners grabbed 1.49m (6.5%), and 1.08m (4.7%) watched Dangerous Drivers on 5 (+1: 102k).
ITV's 8.30pm offering of Nursing the Nation could only muster the channel an audience of 3.04m (13.4%) as well as 133k on timeshift.
Elsewhere, Charlie Brooker's Weekly Wipe slumped to 836k (4.2%) on BBC Two at 10pm.
Overall, BBC One led primetime with 22.7%, beating ITV's 18.1% (+1: 0.8%). BBC Two claimed third place with 5.6%, ahead of Channel 4's 5.2% (+1: 0.8%), and Channel 5's 3.9% (+1: 0.3%).
BARB ratings data supplied by Attentional