Original manuscripts by Bach, Mozart and Brahms form part of a 139-item collection of sheet music donated to the Juilliard School in New York.
The collection includes an early manuscript of Dido and Aeneas
The artefacts - donated by collector Bruce Kovner, chairman of the music acedemy's board - will be housed in a reading room from September 2009.
Highlights include working manuscripts of Beethoven's only opera Fidelio and Liszt's Hungarian Rhapsody No 9.
School president Joseph Polisi said it was "by its very definition priceless".
A lifelong music lover, Kovner began collecting manuscripts more than 10 years ago when he noticed a flow of rare artefacts coming onto the market.
"Great manuscripts were becoming available at prices which I thought were quite a bargain compared to a stuffed shark by Damien Hirst," he said on Tuesday.
Many of the pieces - including an 80-page handwritten script by Beethoven, sold at Sotheby's in London in December for £1.1m - were bought anonymously at auction.
It will be housed in a climate-controlled reading room
Works range from Purcell's opera Dido and Aeneas, written in the 1680s, to 20th Century compositions by the Russian composer Schnittke.
Pieces by Schubert, Schumann, Wagner, Mahler and Stravinsky also feature, often bearing inscriptions and annotations by their creators.
Mr Polisi said it was "one of the finest private collections of music scores to be amassed in modern times".
"The availability of these manuscripts will provide unprecedented opportunities for scholars and musicians at the school and elsewhere for many years to come."