Jeroen Paul Thesseling (1971) followed his first classical violin lessons at the age of seven and started his study bass guitar in 1988 at the ArtEZ School of Music in Enschede, the Netherlands. After playing metal and jazz in his late teenage years he collaborated with the Dutch progressive death metal band PESTILENCE from 1992 till 1994, recording their jazz-fusion influenced studio album 'SPHERES' (1993). From 1995 he experimented with microtonal octave divisions on 6-string bass, which resulted in 'HAFNIUM' - study in 72-tone equal temperament (1999) and 'ARGON' - study in 18-tone equal temperament (2000). In 2005 Jeroen recorded his first fretless work with the world-fusion project ENSEMBLE SALAZHAR. Since that same year he plays exclusively fretless bass.

From 2007 till 2011 he collaborated with the German technical death metal band OBSCURA with whom he recorded the critically acclaimed studio albums 'COSMOGENESIS' (2009) and 'OMNIVIUM' (2011). During the period 2009-2012 he rejoined PESTILENCE after a 15-year break, recording their sixth full-length album 'DOCTRINE' (2011). Since 2011 he plays 7-string fretless basses built by German bass luthier WARWICK, which are strung with a subcontra F# tuning. In 2014 he co-founded the jazz-fusion group SALAZH together with Dutch pianist/ composer Hans Grotenbreg (former ENSEMBLE SALAZHAR) with whom he released the debut work 'CIRCULATIONS' (2017), featuring legendary Cuban jazz drummer Horacio Hernández.

Jeroen has been touring the world. Late 2019 he founded the instrumental progressive metal-fusion quartet QUADVIUM together with American fellow fretless bass player Steve Di Giorgio (TESTAMENT, DEATH). From 2020 till 2022 he rejoined OBSCURA. With them he recorded their sixth studio album 'A VALEDICTION' (2021). This year QUADVIUM finished its long-awaited debut album, set to be released in 2025 via Agonia Records. SALAZH is currently working on their second full-length album.