Kirby's four children have officially requested the rights to popular characters, including The Avengers, Thor and Spider-Man, revert back to the artist's family.
Kirby is considered a comic industry great after co-creating characters such as the Fantastic Four, the X-Men, and the Hulk with writer-editor Stan Lee and is credited with creating the original look of many popular characters.
However, Marvel maintains that Kirby's work for the company was "for hire", invalidating claims made by the Kirby estate.
According to Robot 6, a press release issued by attorneys Toberoff & Associates said Marvel's assertion was an example of a company attempting to deprive a creative talent of rights to their work.
It read: "The truth is that Jack Kirby was his own man. Like so many artists in the fledgling comic book industry of the late 1950s/early 1960s, Kirby worked with Marvel out of his own house as a freelancer with no employment contract, no financial or other security, nor any other indicia of employment.
"Kirby's wonderful creations, which leapt from the page, were not Marvel's 'assignments', but were instead authored by Kirby under his own steam and then published by Marvel. It was not until 1972 that Kirby by contract granted Marvel the copyrights to his works. It is to this grant that the Kirby family's statutory notices of termination apply."
It is believed the Kirby copyright terminations would become effective beginning in 2014 but it is unclear to which property that date refers.