Highlights • 13CDH 3 and 12CD 2 H 2 were measured independently on a Panorama mass spectrometer. • 13CDH 3 and 12CD 2 H 2 geothermometers give CH 4 formation temperature of 150 °C. • Thermogenic CH 4 is not in thermal equilibrium with hosting liquid mud. Abstract Resolved measurements of the relative abundances of 13CDH 3 and 12CD 2 H 2 from a Taiwan mud volcano confirm that thermogenic CH 4 with a formation temperature of 150 °C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium. [ABSTRACT FROM AUTHOR]