Just prior to Christmas, Perry, 27, and Brand, 36, had a "massive fight" over where to go on their holiday vacation.
"She was like, 'F--k you. I'm going to do my own thing.' Russell replied, 'Fine, f--k you too,'" an insider tells the new issue of Us Weekly of Perry, who ended up in Hawaii with pals, and Brand, who spent Christmas catching up with a pal in a pub in chilly London.
Says another source: "They've fought since they met."
Though they denied divorce rumors in November, when Brand told Ellen DeGeneres he was "happily married," the couple were frequently at odds. "Katy doesn't think Russell respects her parents' Christian beliefs or her friends," an insider tells Us.
"I'm married to Katy," Brand told DeGeneres of Perry, whom he wed in October 2010 at a tiger sanctuary in India. "Perpetually, until death do us part was the pledge."
Perry also go into the denial act, slamming seemingly endless reports that she was either pregnant or divorced.
"First I'm pregnant & then I'm divorced. What am I All My Children?!" Perry Tweeted, referring to ABC's now defunct soap. The singer punctuated her post with the hashtags #ericakane #pshhh and #ifihadadollarforeverytime.
Brand finally went through with a divorce filing on December 30, citing irreconcilable differences in papers processed in Los Angeles Superior Court.
"Sadly, Katy and I are ending our marriage," Brand tells Us Weekly in a statement. "I'll always adore her and I know we'll remain friends."
Take a look back at the couple's romance in the video above!