Mont. Code Ann. § 40-2-206

The earnings and accumulations of a married person and of the person's minor children living with the person or in the person's custody, while the person is living separate from the person's spouse, are the individual property of that person except that, to the extent that a mutual duty of support between the husband and wife as established by 40-2-102 still exists, the earnings and accumulations are liable for debts incurred for necessary articles procured for the use and benefit of the married person, the person's spouse, or minor children.