The stability of polyaromatic naphthalene sulfonates in hydrothermal solutions to 330 °C at equilibrium saturated vapour pressure

•The thermal breakdown rate of 2,6-naphthalene disulfonate (2,6-NDS); 2,7-naphthalene disulfonate (2,7-NDS); 1,5 naphthalene disulfonate (1,5-NDS); and 1,6-naphtahlene disulfonate (1,6-NDS) has been measured at different conditions.•1-naphthol, 2-naphthol, 1-chloronaphthalene and naphthalene have been detected as breakdown products of 1,6-NDS at ≥ 200 °C.•The breakdown products of 1,6-NDS can be potentially useful as geothermal tracers.•The assumption of NDS long term stability during a tracer test needs to be done with caution.