He studied music and pedagogy, and had his vocal training with Else Bischof-Bornes and Jakob Stämpfli (song, concert, oratorio) and with Peter Massmann (opera).
[1] In the field of historically informed performance, Klaus Mertens has worked with Frans Brüggen, Philippe Herreweghe, René Jacobs, Sigiswald Kuijken, Gustav Leonhardt and Nikolaus Harnoncourt.
This project, begun in 1994, took ten years to complete with various soloists in the other vocal parts, and included concert tours in Europe, America and Japan.
[9] He performed Bach's cantata Ich will den Kreuzstab gerne tragen, BWV 56, in Wiesbaden[10] and in the Alte Oper Frankfurt, programmed in contrast to Shostakovich's Symphony Babi Jar, conducted by Enoch zu Guttenberg.
[13] He was bass soloist for two concerts of the Bachchor Mainz reviving church cantatas of Wilhelm Friedemann Bach in June 2010, remembering the composer's birth in 1710.