‘This is the hardest episode ever,’ Jon says

updated 2:03 p.m. ET June 30, 2009

NEW YORK - Celebrity parents Jon and Kate Gosselin say they plan to divorce.

The co-stars of "Jon & Kate Plus 8," who are parents of 5-year-old sextuplets and 8-year-old twins, spoke of their decision to separate during Monday's episode of the TLC reality series. Near the end of the hour-long episode, a title card summed things up in stark terms: It said that on Monday, "legal proceedings were initiated in Pennsylvania to dissolve the 10-year marriage of Jon and Kate Gosselin."

The network had been promising a major on-air decision from the couple, whose increasingly troubled marriage has dominated the series in recent weeks, as well as fueling a firestorm of tabloid coverage.

"We've been dealing a long time with this," Kate said, speaking of her marital problems and the attendant publicity.

"It's been a hard seven or eight months," Jon said in one of his separate on-camera sessions. "It's been a really hard last four months. This is the hardest episode ever."

The beginning of the program was taken up with the delivery and assembly of prefab playhouses for the Gosselin kids. It was the sort of family activity that had formed the crux of the series, now in its fifth season, as it was originally envisioned: A reality show that would chronicle the challenges of raising a supersized family.

‘Jon & Kate’ video   ‘Jon & Kate’ announce split June 23: Msnbc.com's Courtney Hazlett has the scoop on the married stars of TLC's "Jon and Kate Plus 8" announcing they are separating.   What's next for the Gosselins?   ‘Jon & Kate’ make it official

But even during this interlude, Kate and Jon were barely speaking. Where to install the four playhouses was a source of friction between them.

"My stomach is churning because I know this is gonna cause a huge fight," Kate told the camera as the installation process began in their yard.

The latter part of the show focused on the couple as they discussed the painful decision they have settled on.

"I just need relief now," Kate said. "I need to turn the page."

Jon described his relationship with Kate as "a total breakdown of everything."

‘I don’t want to do this alone’
"Jon & Kate" is TLC's most-watched series, drawing nearly 10 million viewers for its season premiere recently. But with the couple's celebrity has come nonstop celebrity coverage, and scandal. There have been reports of infidelity by both Jon and Kate (accusations both of them deny).

Adding to the strain, Pennsylvania's Labor Department opened an investigation into whether the show's production is complying with state child labor laws. TLC has said it "fully complies" with state laws and regulations.

Looking ahead, Kate said, "I don't really want to be alone. I don't want to do this alone."

But she described the split as "the next chapter." And while she and Jon have spoken angrily about the pressure on their marriage from unsought fame and press attention, she conceded, "I believe that it's a chapter that probably would have played out had the world been watching, or not."

One thing the couple could agree on: "Jon & Kate Plus 8" will continue.

"We interview separately, we'll film different things," Jon explained. "Me and the kids, her and the kids."

"How does the show go on?" said Kate. "The show MUST go on!"

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