University Librarian Alice Prochaska announced Friday that she will resign next year to become principal of her alma mater, Somerville College at the University of Oxford.

The appointment is effective September 2010.

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Prochaska is an alumna of Somerville and received both her undergraduate degree and a doctorate in modern history from Oxford. Before coming to Yale in 2001, she served at several British institutions, including the British Library, where she was director of special collections, as well as the Institute for Historical Research at London University and the National Archives.

“I am honored and delighted to have been chosen as the next Principal of Somerville College,” Prochaska said in a statement. “I am sad about leaving Yale University Library, where I will have worked for nine years with a wonderfully talented and dedicated group of staff, but I know my colleagues will understand the tug of affection and loyalty that I feel in returning to Oxford.”

University President Richard Levin praised Prochaska on Friday for her accomplishments over the past eight years, which included a significant push toward digitizing many of the library’s resources and the $47 million renovation of the Bass Library.

“This is a tremendous honor for Alice, and although we will miss her at Yale, her new position provides her with a remarkable opportunity for leadership at her alma mater,” Levin said in a statement. “During her eight years as our University Librarian, Alice has provided conspicuous leadership in advancing Yale’s library system — although she would be the first to say that it is not her work, but that of the Library staff that should be credited.”

As university librarian, Prochaska oversees the 22 individual libraries that make up the Yale University Library. Altogether, the library system employees some 600 staff members and, with approximately 13 million volumes, is the second largest academic library in the world.

Prochaska, who will go on research leave in January before assuming her new post the following autumn, will become the second top administrator to leave New Haven for Oxford in as many years. Former Provost Andrew Hamilton will become Oxford’s vice chancellor in September.

Somerville, which has an enrollment of 500 students, was founded in 1879 as a non-denominational college for women and began admitting men in 1993.