Expressions are given for the tritium distribution between gas and liquid phases arising from prolonged electrolysis of D2O on Pd electrodes saturated with absorbed deuterium. Errors associated with commonly employed experimental procedures and their interpretation using a simple model for the distribution between the gas and liquid phases are evaluated. With regard to tritium production, an unambiguous resolution requires a complete mass balance. Conclusions based on partial data, such as the measurement of the tritium content in a single cell component (e.g. electrode, electrolyte or "electrolytic" gas phase), may be misleading, particularly if production rates are either low or intermittent.