Claude Janssens studied classical trombone with Guillaume Ackermans (former professor at Liège school of music) and jazz trombone with Jean-Pol Danhier (professor at Brussels school of music). The saxophone was, at that time, his second instrument.

From 2000 to 2013, Claude Janssens devoted himself entirely to his responsibilities as director, then CEO of "La Médiathèque" (a non-profit network of media libraries in the Walloon Region and Brussels.)

Since his comeback in 2013 Claude Janssens mainly plays bass clarinet, the flugelhorn and the trombone being his second instruments.

Regarding his composition work, CJ is essentially autodidact, though he did obtain a Master in musicology at the University of Brussels in 1980, and undertook a study of contemporary musical analysis with the composer Karel Goeyvaerts.

From 2013, he has released two albums uitgebracht "Rituals" with The Janssens Flexible Ensembleet and "Instants" with his band Close up 5.