Molecular mechanisms of solid bitumen and vitrinite reflectance suppression explored using hydrous pyrolysis of artificial source rock

•Vitrinite and solid bitumen reflectances are suppressed in rocks with high amounts of liptinite.•Vitrinite and solid bitumen have slightly different suppression mechanisms.•Raman proxies indicate lower aromaticity in samples with suppressed Ro.•µ-FTIR proxy CH2/CH3 indicates reduced bond cleavage in vitrinite with suppressed Ro.•Data support mechanism of slowed aromatization due to free radical generation during maturation.