Bass-baritone Klaus Mertens is celebrated by the press for his "unique expressiveness, his pleasant timbre, his sensitivity for words as well as his convincing music-making" and for being an "excellent master of his Fach". "His interpretations of songs, cantatas and oratorios set standards."

Klaus Mertens’ unique characteristic is that he is the only singer worldwide to have sung and recorded the entire vocal oeuvre of both JS Bach and Buxtehude. These recordings, directed by Ton Koopman, continue to enjoy the highest international recognition.

As "one of the leading Telemann singers of our time", Klaus Mertens was awarded the Telemann-Preis of the city of Magdeburg in 2016. "As the ideal conveyor of Bach’s cantatas and passion texts…" he received the renowned Bach-Medaille of the city of Leipzig in 2019.