Our well-placed sources -- and they're solid -- tell us Scott hasn't come home to the family house in Calabasas in over a month and Kourtney lost her patience.
As one source put it, "His partying lately has made her make this tough decision."
We're told Kourtney is also frustrated with Scott's friends -- and believes they're enabling his substance abuse.
Scott and Kourtney have been together since 2006. They have 3 kids together.
It's well-documented that Kourtney and Scott have been rocky over the years -- but we're told, "but now, after 3 kids, it's gotten old and she has to do what's best for the kids."
As for Scott, he was last spotted in Monte Carlo -- getting touchy-feely with celebrity stylist Chloe Bartoli.
Sources tell us Scott knew the relationship had ended -- because he was telling people in Monte Carlo that he was "newly single."