The payment will come from a mixture of cash, Ancestry.com stock and assumption of iArchives liabilities.
The move means to offer greater content, as well as enhanced digitization and image-viewing technologies.
The deal has not been completed yet, but when it is, iArchives will be a wholly owned subsidiary of Ancestry.com. The final details to cement the transaction are expected to take place in the coming months.
iArchives specializes in historical records of individuals who were involved in major milestones in American history, such as the Revolutionary War, Continental Congress, Civil War and others. The firm has digitized more than 65 million original sources.
iArchives operates Footnote.com, a subscription website that offers searchable original documents that describe the people, places and events that are all part of American history. In addition, Footnote.com invites individuals to discuss and collaborate on what they have learned with family, friends and colleagues.
"By promoting Footnote to our Ancestry audience, we hope to expand its reach among researchers who care about early American records. iArchives also brings outstanding image-viewing technology and content digitization capabilities that will improve our leadership position in bringing valuable historical records to the market. We welcome the iArchives team to the Ancestry.com family," Sullivan said.
Ancestry.com, which has added more than five billion records to its site in the past 13 years, has about 1.3 million paying subscribers. Those who use its services have created more than 19 million family trees that contain more than 1.9 billion profiles.