The performing parts for the premiere of Beethoven's Mass in C had received little attention before the new complete edition in 2003 detailed their existence and contents. In his 1959 article “Beethoven in Eisenstadt” Johann Harich confirmed the existence of forty-four parts then in the Esterházy archives and mentioned their untidy appearance: “The number of entries, corrections, and interpolations in his [Beethoven’s] own hand in the parts is enormous. There is not a single one in which such things could not be found.” The parts are now in fact complete, since the Esterházy holdings can be supplemented by two later discoveries: The National Széchényi Library, Budapest, houses four additional choral parts, which include the important master copies for the tenor part and a portion of the bass part; and James Armstrong found the missing timpani part in Eisenstadt while preparing his yet-to-be-published catalogue of the Esterházy music archive. Forty-nine parts now exist, a number which agrees with an inventory of the Esterházy holdings made in 1809, just two years after the premiere. These parts, along with what remains of Beethoven's autograph manuscript, together with a complete score (delivered with the parts to Prince Esterházy in 1807) and the first edition published by Breitkopf & Härtel in 1812, constitute the extant primary sources for the Mass in C.

Of these, only the parts can help us understand what happened at the premiere on September 13, 1807. Beethoven, at least, thought it went well, or so he reported in 1808 when he offered the work to Breitkopf & Härtel for publication: “Also it has been performed in several locations with great applause, among others at Prince Esterházy's in Eisenstadt for the nameday of the princess.” Two eyewitnesses recorded other reactions. Joseph Rosenbaum, a former official of the Esterházy court, noted blandly in his diary: “[T]o Mass … with unsuccessful music by Bethowen [sic].” A more damning criticism came from the prince himself, when he wrote to Countess Zielinska: “Beethoven's Mass is unbearably ridiculous and detestable, and I am not convinced that it can ever be performed properly; I am angry and ashamed.” Whether because of the perceived novelty of its style or the musicians’ inability to perform it properly, the premiere clearly did not have the intended effect.