Highlights ► Dithiocarbamates degradation products ETU and PTU are determined at sub-ppb levels in water. ► Rapid ETU and PTU determination by direct injection in UHPLC–MS/MS QqQ (LOD 0.058μg/L). ► Alternative on-line SPE-LC–MS/MS can be used for ETU in water samples (LOD 0.027μg/L). ► Strong matrix effects observed are corrected for ETU using its own deuterated analyte as ILIS. ► Non satisfactory correction for PTU using PTU-d6 reveals that analyte ILIS may have different chemical behaviour.